In late game, when you are superiour to any one of the competing civs they will feel that you're getting close to winning the game while at the same point realizing they're not going to win themselves. At that point, they will do anything to stop you from winning, even though that may mean the complete and utter destruction of their own empire. They're not actively ganging up against you, but this decision is made by each of your competitors individually. It seems that an AI player rather helps a competing AI to win the game at the cost of its own live than to allow a human player to win.
I don't have any numbers or other facts from within the engine to prove my thesis, but consider this:
- The AIs will get very forgiving towards each other and will even settle long-running wars between each other in order to more effectively battle you
- They will crawl across the whole map, leave their own cities dangerously low defended, engage in battles with ridiculously low chances of success, and sometimes declare war upon you without even having a marching route into your territory
- In multiplayer matches The AIs will stay their armies until you are the last human player standing
If these aren't strong signs for an AI conspiration against humanity I don't know which are…