Ideologies can also cause huge unhappiness, if you dont pay attention to your tourism and culture.That's why I sometimes DOW on the civ which ideologies cause me severe unhapiness.
Also after the fall patch, I feel that the AI really take consider to DOW on player if we almost reach certain victory condition.
I once play peaceful science Korea with one city, no border problem, never denounce other civ, always let them get away when their spy stole all my techs.
But when I finish the 3rd part, the whole planet seems to DOW on me. So I have to sell all my building and rush buy the remaining part.