What I think it was is, the game thought I was on the pit lane, and when I got too far from it, it pushed me back. You can see my right back tyre slipping into the pit lane and the camera changing automatically (as it does when you enter it and you're not in first person view). And poor AI-guy was right behind me and was pushed for a quick refuel, on the last lapHe did overtake me though, and finished 15th.