I just played a game where an A.I. Lumeris empire was defeated out of nowhere. The other factions were not attacking them and they had the second largest empire in the match. No other empire was attacking them but me. I only attacked ships that passed through my boarder and never ventured into their territory. When I was taking over a Vodyani A.I's territory on turn 120 the Lumeris empire completely collapse and disappeared. The score screen showed that the Vodyani empire was still alive when it wasn't and that the Lumeris empire was dead. It was surprising to say the least. 

I just scored a military victory... without siege and capturing any planet of the other empire