I was playing my first single player game in GalCiv3, I had four A.I. allies and eleven A.I. enemies. by turn 285 I'd spent over 16 hours on the game, and I had first place in influence, population, research, construction, and number of colonies as well as fourth place in military power. I noticed something strange that would occasionally happen where a popup would say "Such and such civilization has been defeated" and then the next turn all of their colonies, ships, and starbases would belong to another civilization. I couldn't figure out what was going on. I tried attacking the capital planet of an enemy civilization, but all that happened was I got that planet and another planet became their capital, so that' not how defeat works. Then I tried just building a bunch of culture starbases all around an enemy, but I annexed two planets and then nothing else happened, so that's not how defeat works. Then, very suddenly, the Slyne were defeated by my allies and the next turn, a cutscene played showing planets being destroyed and I was informed that I was defeated. I could not continue to play, and the deta and graphs the game gave me didn't inform me of what the hell happened.
So my question is simple: How does defeat work in GalCiv3?