I'm sure it could be any number of things. one of the wars was just so random. I wasn't disliked at all in fact I was on the positive side of neutral and separated by a majority of the continent with no real contact and two other civs in the middle.
seems very hard to stay on anyone's good side. Going through the diplomatic screen and clicking each leader they are all hated by each other on my current game. Mostly just a bunch of red frowny faces.
When I managed to be on someone's good side they end up back stabbing me. Fuck you Teddy Roosevelt!