Deater,
I have the same thing with Romans. I was appeasing them through the whole game (as Babylon) and suddenly they declared a war on me. And just like in your case, it was not some petty skirmish, but a full scale invasion.
I don't think this is an AI bug though. It is nice to be a peaceful and culturally advanced Civ (I like that style very much), but historically and realistically speaking, your civ is only as good as the army defends it
(In real world history, Romans conquered culturally advanced Greeks and Egyptians, despite the fact that the latter did not provoke them too much.)
Also, what quite surprised me was the Roman AI skill at handling the invasion. Rather than conquering and keeping the city (which would make it revert to me in no time, due to my cultural superiority), they razed it and moved all citizens (as slaves/workers) into their Empire. I must say I was quite impressed. Only within few turns (involving quick change to Monarchy - Religious CSA coming in handy) I was able to get enough advantage, and Romans offered me peace in exchange for the workers they enslaved (another nice detail making the game great :b)
So, all in all, as the ancients Romans put it: "Si vis pacem, para bellum" - "If you want peace, better prepare for war"
