Well, this happened often to me but something strange happened. I was playing India and I was fighting a war against Kublai Khan (damn backstabber...) over centuries. I had two friends, Elizabeth and Boudicca, which couldn't stand Kublai either. They didn't want to join the war, so I had to do it myself. But once, as soon as I hit enter, everyone declared war on me! There weren't any pacts or such, the Apostlic Palace has been built (but I didn't know the owner).