Ngtm1r's Shivan theory
The Shivans are not drawn to subspace disturbances, though they are attuned to them.
The Shivans are drawn to conflict. Perhaps they believe strongly in peace: strongly enough that they are ready, willing, and able to enforce it on those who are unwilling to be peaceful. For those who attract the attention of the Shivans receive the most absolute and final form of peace imaginable: that of being dead. Or maybe the Shivans just like a good fight.
The Shivans slaughtered the Ancients, but only after a long period of time in which the Ancients were slaughtering other folks. This would seem to contradict my premise, but note that I said the Shivans were drawn to conflict, not violence. That wording was intentional.
According to the Ancients monologues most of the species the Ancients wiped out didn't (or couldn't) put up much of a fight. ("They were like all the others, hideous, resisting, fighting. Only they were not like the others. They did not die.") I can only guess, but maybe a little while before the Shivans came along there was a species that did put up a fight, not enough of one to stop the Ancients, but enough to bloody them.
In come the Shivans, drawn to the conflict, to destroy the combatants. They only find one standing, and wipe them out. The Ancients were not a peaceful race. At first they thought the Shivans were the wrath of an angry cosmos...but not all, apparently, since they kept looking for a weakness, and they found one eventually. They learned better.
Left alone, the Ancients would have expanded again, perhaps now with even greater xenocidial tendencies thanks to the Shivans nearly wiping them out. And when the Shivans sent a new expeditionary fleet to stop them, the Ancients would have possessed the means to destroy it. Clearly they had no intention of rolling over and playing dead. The Ancients would be back. The Shivans could be beaten, they were not the gods the Ancients first thought. ("We have little left. Little time. But much irony." "In subspace their shields will not function, and into subspace they can be tracked.")
8000 years later, we have the Terran-Vasudan War. It's a long one, 14 years, and so I would guess it was not a high-intensity war for most of that time. Around the time the Shivans show up, however, it had clearly escalated to a high-intensity war. (Look at the casualty figures for Operation Thresher, for example. That's a lot of people considering they're pilots. Had they been infantry, I wouldn't have batted an eye, but five hundred pilots is an awful lot to lose in one operation.)
And here come the Shivans again, determined to enforce a final solution to the question of Terran-Vasudan relations. Only they fail. The Terrans and Vasudans unite against them, and destroy the Shivan expeditionary fleet.
But, this may well have been viewed by the Shivans as a victory. They did not immediately withdraw once the Terrans and Vasudans had made peace with each other because that peace would have evaporated almost as quickly as it had been created. So the Shivans kept fighting. If they ultimately won, there would be a final, total peace. If they ultimately lost, but made a good fight of it, one that forced the Terrans and Vasudans to work together to the fullest, to support each other economically, militarily, and socially, to integrate, then the peace between the Terrans and Vasudans might well last.
32 years later, the NTF Rebellion breaks out. It does not attract the Shivans' attention, however. Perhaps it would have eventually, but there is a short-circuit of that process. 17 months into the NTF Rebellion, the NTC Trinity activates the subspace portal in Gamma Draconis and enters the nebula.
We actually don't know who fired first in this case. We do know the Shivans fired first on the GTC Vigilant, but we do not know if the NTC Trinity fired first on the Shivans. I find it highly probable the Trinity encountered a Shivan patrol, which closed to look over this Terran ship that had somehow entered Shivan-controlled space. The Trinity, seeing Shivan ships and knowing what the Shivans did the last time they were in the neighborhood, interprets this as the Shivan patrol preparing to attack, and opens fire.
Now, the NTF Rebellion hasn't reached the level of conflict yet where the Shivans decide to intervene. But if you're going to show up in their space and attack them, they're going to respond in kind. The Shivans dispatch a probing force through the subspace portal to take a look around, set up a base camp on the other side, and in general prepare for a sortie. The Shivans, however, do not intend to attack the GTVA. They have no reason to, yet. The Shivans are out to get the NTF. Only they probably can't tell those two factions apart very well, if at all.
The first Rakshasa into Vega encounters the GTC Vigilant. The Rakshasa, encountering what is probably a Terran ship, attacks, not realizing that it is attacking a GTVA ship and not an NTF one. After all, the Trinity had to come from somewhere, and Vega is the most likely candidate (Gamma Draconis was uncolonized and empty).
Now the GTVA knows the Shivans are back in town. And considering that both of the member races of the GTVA lost their homeworlds because of the Shivans, from the moment the Rakshasa fired on the Vigilant there was no chance of a peaceful resolution. There was little chance that the Shivans and GTVA could have come to a peaceful resolution before the Rakshasa attacked the Vigilant.
The GTVA now must do something with that information. They choose to act on it, swiftly, decisively. The Shivans were not all that powerful in the last invasion, only a few destroyers, a bunch of cruisers, and a lot of fighters. (In fact, probably only three or four destroyers: that's all Terran Command seemed to think that there were in FS1, and if they had many more then that how the GTA and PVN weren't destroyed before they could get their act together is hard to comprehend.) The GTVA's military arm was built and trained for just this moment. There is no hesitation, and the order is given: Attack!
The Shivans, meanwhile, may or may not have realized their error in attacking the GTC Vigilant. At this point, it does not really matter. What they did not expect was the speed and power of the GTVA's counterattack. Vega Command hears the Vigilant's distress calls and tells the Carthage and Dashor to drop everything and head to the Gamma Draconis node. The Carthage and Dashor arrive on the scene at most a few minutes after the Vigilant is destroyed, perhaps less. The Carthage deploys fighter and bomber wings. Both it and the Dashor move to engage. The Rakshasa and its escorts never really have a chance.
This engagement gives the Shivans pause. The GTVA's ships are not the equal of their Shivan equivalents, but they are close, close enough that the difference is not insurmountable. The GTVA, meanwhile, is executing its preset war plans for second contact with the Shivans: attack with all available force, at once. They are confidant in their knowledge of Shivan ships and tactics, have trained and prepared for this battle for 32 years. They will prevail.
And prevail they do. Before the Shivans can fully digest the implications of the engagement at the Gamma Draconis node, the GTD Aquitaine arrives in Gamma Draconis and engages Shivan forces there. The Shivans are unprepared, and are driven before the Aquitaine's battlegroup or destroyed. The Aquitaine, moving at blitzkrieg pace, pushes on through the portal and engages Shivan forces in the nebula, pushing them back, inflicting heavy casualties.
The Shivans are shocked. This has never happened before. Not only has their planned offensive come a cropper before it ever got underway, a first in and of itself, but the offensive has actually been turned around on them as the GTVA pushes into the nebula.
Then suddenly, it stops. The GTVA stops pushing and merely starts to hold. The Shivans are mystified, but use the breather constructively, dispatching a Sathanas and supporting warships to push the GTVA back out of Shivan space and then end this fight in the usual Shivan fashion of annihilating all opposition. In reality, what the Shivans don't know is that the GTVA is in the process of dealing the deathblow to the NTF.
The Sathanas is coming, but it isn't there yet, when suddenly the GTVA explodes into action again, pushing harder then they had before, throwing more ships and firepower into the attack then before. The Shivan front line crumbles, and rear-area Shivan operations are opened to Allied attack (the gas miners you hit in your first Vasudan mission). But all is not lost: the Shivans finally get the Sathanas into the area, and as they hoped GTVA opposition crumbles before the might of the juggernaut.
Then the battles at either end of the Gamma Draconis-Capella node result in the destruction of the Sathanas, and suddenly the GTVA is back, pushing hard yet again, trampling the small Shivan force still in the nebula. The Shivans decide to throw the book at the GTVA. More Sathani are dispatched.
Something odd is happening too, though: one Terran ship is not attacking them, but trying to communicate with them. The Shivans are mystified, but agree to meet Bosch and talk with him. The combat aboard the Iceni was a colossal miscommunication, a panicked crewer firing on the Shivans that boarded and the situation escalating badly before Bosch got through to both the Shivans and his crew. There were, after all, two transports that docked with the Iceni: if Bosch and company, plus the Shivan boarding party (or what was left of it) were onboard the first, who was getting on the second? Probably a good portion of the remainder of the Iceni's crew. ("As I make this final entry my crew is preparing to scuttle the Iceni and board the Shivan transports.") But starships were not made to have firefights inside their hulls, and damage was done to the Iceni's systems. Damage that left Lieutenant Rusk and his compatriots in the dark about the peaceful ending to this encounter, although they probably wondered why the Shivans hadn't returned to exterminate them.
From their discussions with Bosch, the Shivans learn that they have made a giant mistake, starting with destroying the GTC Vigilant and moving on to every other GTVA ship and crewer or pilot they have killed since. The GTVA is acting out of an apparent need to defend themselves, not a desire to attack the Shivans. The ones who started it were the crew of the NTC Trinity: even the NTF, in the personage of Aken Bosch, never meant for there to be a war with the Shivans.
But mistake or not, the Shivans have thrown the gauntlet down, and the GTVA has picked the gauntlet up. The GTVA will never make peace with the Shivans. Rather, the GTVA will make every effort to annihilate the Shivans just as the Shivans have annihilated so many others. The Shivans no longer wish to destroy the GTVA, though. The GTVA has done nothing wrong. So somehow, they must end this fight without a formal peace.
The GTVA has Knossos technology: if the Shivans merely retreat and blow the nodes leading into the nebula, the GTVA will believe they are winning, use Knossos portals to reopen the nodes, and continue their campaign. Destroying Gamma Draconis might work. But it might not, as the GTVA might conclude it had cost the Shivans much of their Sathanas fleet, and the rest had gone somewhere else or been heavily damaged. Gamma Draconis hardly matters to the GTVA, after all. There's nothing there. (The system is uncolonized; the last significant attempt to survery it was fifteen years ago.)
Finally the Shivans settle on causing the Capella supernova as the best, most clear way of both sealing themselves off from GTVA incursions by that route, and sending a message: Don't screw with us. We're bigger, we're meaner, and we will kick your ass. And they carry this plan out.
The GTVA doesn't comprehend the reasoning behind what happened, but the message got through loud and clear. They know better then to screw with the Shivans now.