A friend asked me this years ago and my completely unfounded hypothesis was that it was due rather to a whiplash effect. A person, held in by a seatbelt and suddenly having their forward momentum impeded or halted by an impact would sustain a great deal of force. I thought this force would be dissipated along the length of the body and terminate at the feet rather like a whip cracking thus the shoes fly off. Am I even close? 
Hutchison effect
that’s my story and I’m sticking to it.
The loser they are, the better they fly. No-heel sleepers are best, high boot do not fly at all…
Well, I once saw an accident scene that was right off the parking lot where I worked. Two guys were crossing an intersection with a green light when a drunk driver plowed into them.
Their shoes where still in the crosswalk while their bodies were about 150’ down the road. They had been knocked out of their shoes and carried/thrown by the pickup truck that hit them.
Maybe not what you’re talking about with victims in a car but freaky shit just the same–especially since if I hadn’t stopped to chat with some people on my way out after work, I would have been riding my bike down that road at that same time. This guy was so shit faced he was bouncing off the telephone poles as he flew down the road.
We had a pedestrian hit by a car recently, and her shoes were left standing where she was hit. Knocked her right out of 'em. I have no idea why, I just wanted to share that.
Inertia…
Peace,
mangeorge
Well, as to the people inside of the car…
Probably because one of them was going for, or had finished the big nungamunch just before the car impacted.