I am not sure what causes this. But you ever notice how when you agree to peace with an AI, it still sends units after you sometimes 2 or even 5 turns after you made peace? And they will capture or destroy whatever in your path. Is this a bug, or related to chain of command? If this is part of the game for whatever reason, what work around is there to defeat such a mechanism? It sucks when I declare peace just to save a city, I give him all my techs for peace, yet he still attacks it anyway!