A) parking brakes hardly work when the vehicle is going backwards (not sure about tow trucks though...).

B) If it is all wheel or 4 wheel or whatever you want to call it the other drive wheel could be on the other side that we can't see. So it could just be spinning?

Not trying to argue that I think it really happened but that it is plausible that this could happen.

I also hate people that just say the first thing off the top of their head when they really know nothing about the subject at hand.