Even though I like my errant bobsled theory, I think Caveman landed on it. I can't get my head around not hearing the striking of some animal, but creeping up on a pile of snow while parking...
It'd be gentle, quiet, cause the shape pictured, and you'd likely attribute the loss of forward momentum to loss of traction.
Look at the car in the background in the 1st pic. It's likely got a matching dent.
