Did you ever have a random CTD during the AI's turn? Logically, the AI is doing something that the game doesn't like, and so the game crashes. When you load the game back up, since the AI behaves differently in a reload, it will do something else (presumably not whatever it was that made the game crash) and thus not crash. Am I making sense with this?

PS. CA reps say that they have "improved" the the bug (I don't recall them saying they actually 100% fixed it). My theory is that since they (probably) didn't change a huge amount of the structure of a savegame file, they probably just programmed the AI to keep any sieges they have standing upon reload (so the other consequences, like weird diplomacy etc would still be in). Again, I'm probably wrong, but hey, I could be right.