This reveals an important point that is rarely discussed in the conversations about the failings of the AI. In light of the 'boardgame' design, where civs are rivals in a game competing to win, I think there is much more emphasis in this game than there was in civ4 on 'taking care' of the guy at the top of the scoreboard. It's like a jealousy factor. If you're small and weak, it's easy for bigger neighbours to pick on you and take your cities. If on the other hand, you're leading in score, the other AIs want to slow you down. It's possible to find a place somewhere in the middle, but it requires a particular playstyle and I think a lot of people who are severely disappointed with the AI and can't see any rationale to any of its decisions are not playing in that middle playstyle.