Looks like a pretty typical pedestrian PI. On impact, the body goes over the vehicle (or rather the vehicle goes under the vaulting body). The body rotates, end over end quite violently. It is the centrifugal force of this rotation which causes hats and shoes, glasses and pocket contents to fly off. Decapitation is quite unusual, but definitely possible at very high speed, say over 100 mph. I have worked on a couple of hundred fatals easy over the last 40 years. Very time consuming to pick up all the pieces and make the measurements. I don't do the math calculations, not my specialty.
The worst was when a crow flew down and started eating a guys brains off the road. Fatal accidents are not pretty.
