I think it has to do with the races. If you start a race and then quit out of it, the rear of the vehicle will face you. If you do a race the front end will be facing you. I guess it is a way of showing shame or something.
It's when you lose (backing out of a race being the same thing). I go back to my garage a lot, and the only time I saw my rear bumper there was when I accidentally entered a race way over my head and decided to run it for fun, but lost.