This may be completely incorrect, but
I think all the saths were going to jump to another part of the galaxy or another galaxy with a super-node. They activated a prototype technology and all of a sudden the sun went nova, they made an in-system jump to gamma-draconis and warped out as fast as possible, some of the saths tried to warp to theire destination but it didn't work and they were taken by the nova, it was a premeture star (or the star wasn't big enough, or both) and their hulls maintained together for a few seconds (but all life inside was extinguished) and would have blown up, but before that happened a great big subspace eruption blew a tremendously powerful explotion that incinirated the entire ship in less than a second. Only 20 or so of the ships survived, the super-nova was far to quick for all the ships to warp to gamma-draconis and the nova reached them.
No saths are in Capella, the nova has since receeded into a nebula, though still hot, and in years to come, they will send a force here to see what happened, and start mining the nebula gas. Command may open the portal with a knosses device and the ships that travel their will get continuous (though maybe not suvere) damage from the heat (maybe like a maxim firing continuously, or an Lred) and won't stay long.