1. I would found a city
2. the city didn't ever have happiness problems
3. it had plenty of food coming in
4. none of these factors changed

and, within four turns, I got two messages at the start of a turn, saying:

"City ________ is rioting"
and
"_____ has been destroyed by famine"

Since the cities shouldn't have rioted, and there was plenty of food, what could have caused this?

Also, occasionally, for no reason, four or five of my cities would riot, even though they had more positive happiness than negative.

Are these known bugs? Is there a way to fix them?