Capellan barbecue theory
As related by Eishtmo
The Shivans, you see, are the universe's greatest party animals. Their parties aren't just legendary, but virtually mythic. The only problem is that they lack one very vital thing: Beernuts.
They've got plenty of beer, massive ships haul it back and forth from the alcohol nebulas scattered across the galaxy, processing them into the golden brew that every Shivan loves to drink by the keg. That's a keg every round for those who are concerened. Beernuts, however, are in painfully short supply and the Shivans have sent out massive fleets of ships to locate a rich supply of those salty little buggers.
Then they found the GTA and the Vasudans.
The Shivans, despite their apparent distructive nature, only wish to spread their partying ways across the universe. Their laserpointers and bottlerockets, however, rip starships appart and kill people who are not Shivans (to whom, getting hit with a laser blast is the equivilant of a come on). Not to be detered, the Shivans kept at it, until they learned that these strange creatures that exploded when a Shivan offered them a night of passion had a massive supply of beernuts.
Then the horror began. Led by the lead party barge, the Lucifer, the Shivans made a mad, highly destructive (sometimes by accident, sometimes not, they are harty partiers) search for the source of the nuts. At one point they thought they had it, and in celebration, fired off a massive firework display at Vasuda Prime, utterly destroying the surface of the great planet. After a careful search, they found no nuts, but they did find that the nearby Terrans DID have nuts, and made a beeline to Sol to claim them.
They didn't count on the Terrans not wanting to have their planet be part of a firework display, and so the Lucifer was stopped and the rest of the Shivans, their party supply cut off, wandered off to pass out in a galactic ditch and sleep it off.
Bosch, a very wise man who partied harder than three frathouses during homecoming, came upon evidence of the Shivan's true intensions and set out to join them. Using the Knossos and the NTF as cover, he sent the Shivans an invitation to a truly massive party. Luckily, the Shivans had found a rich source of beernuts and decided that they should set up a truly excellent party amongst the GTVA systems. The Sathanas (which is really just a massive party barge/grill) made it's way through Capella to find a good place to set up. When it ran into the Colossus and was destroyed, the Shivans figured that they should set up the party closer to home and launched the fleet.
With Bosch drinking at their side, the Shivans gathered their great fleet of grills around Capella and they blew up the star to roast lots of meats and veggies so the party could begin proper. They were quite disappointed when most of the Terrans and Vasudans didn't show up, and in fact slammed the door shut on the party, but not being ones to take offense, just took more for themselves.
And so the great Capella Barbeque began, and the Shivans and handful of humans they managed to find rocked hard and long all night long.