My theory on why the Capella Star blew up is this:
The Shivans, having spent nearly three days pumping some kind of subspace field around the star, were powering up a weapon as Petrarch suggested. It could have been many things, but I believe the Shivans were simply sick and tired of interference by the GTVA. This subspace field rippling from the star could have been anything, and I do not have suggestions as to what its intent was, but it surely didn't mean good times for the GTVA . I surmise the Capella Incident was a calculation error on the part of the Shivans, because of the destruction of the Epsilon Pegasi node minutes before. Had the node not been collapsed by the Bastion, it is very possible - as the Shivans were playing around with subspace around that star - that their plan would have worked, and the consequences might have been much more catastrophic.
I mean...why the hell would the Shivans have been so persistent in taking out the Bastion otherwise? Yeah it's a Terran destroyer with escort, but it was escaping the main battle, so it shouldn't have drawn that much attention, but even with the Colossus fighting at the Gamma Drac node, to quote Petrarch "the battle group protecting the Bastion has been decimated" The Shivans most likely knew that the Bastion carried Meson warheads and deducted they wanted to repeat history by destoying a large ship inside a node to collapse it, therefore ruining their own plans and risking failure of whatever they were doing with Capella.
The Outro shows the Shivan juggernaughts closing in on the star and creating some weird subspace field, which might have been simply to DELAY the unintended destruction of Capella while most of them fled to God knows where.
At least that's what I think.