Obviously no one really knows what happened except those people in the room, but I don't see any reason to doubt that he quit of his own volition. I do happen to like 8, but that's beside the point for now. With little news on how good or bad the OS is doing, and the fact that official numbers won't be out for a little while longer, MSFT would be stupid to try to push this guy out now, which would obviously start rumors of 8 doing very poorly, and therefore hurt sales further. They would want to hold him on for a while longer until just before the official numbers, definitely after the holiday shopping season, to avoid a sales hit. Of course, someone wants to quit there's not much you can do to stop them. I think he more likely quit than was forced out.