*Coming up with Shivan Error Theory as I write this.*
The Shivans were bombarding the star with subspace waves, then at the end they make various strange subspace rings or portals or something. Maybe they were attempting to channel the energy of the star into subspace, the reason for the small pulses was to get the star into a synchronous vibration with subspace, thus making it that much easier to transfer the energy, or perhaps taking the entire star into subspace. But the Shivans made a miscalculation, and the sathanas portals collapsed, perhaps sending a cataclysmic subspace pressure wave straight to the core of the star. The balance between gravity and radiation is upset, the star partially collapses on to the core and rebounds in a massive explosion. Most of the Sathanas fleet was able to escape while a few stayed behind to keep the star in check long enough for the rest to leave.
It may be completely unreasonable, but Dark Hunter's idea intrigued me.