I'm still a little new at this game, but I've noticed that whenever I go to war and start successfully invading alien planets, the AI always surrenders before I can conquer their last planet (or two).
Basically, once they're undeniably beaten, they give their planets to another AI. The only AI player that has fought "to the death" has been the Altarians.
Is this normal? It's kind of annoying, but at the same time, I see the logic in it. If you're crushing one civ, it makes sense for the game to make the next one a little harder to beat. So, I'm not really complaining... I'll just have to learn to coordinate my attacks better, in order to conquer all the enemy's planets in one turn.