Somehow the beginning relations got screwed up. It happens sometimes, and I don't think anyone really knows how or why.
To fix it, start another game; when you get to the screen to pick your opponents, select each one in turn and set their starting relations to "unknown". That should clear it up, but you may want to select ALL opponents, even the ones you don't plan to have in that game. These probably got messed up as well, and if you ever play against them their relations will be screwy too.