Sorry about your terrible accident.
I think it was freak incident. I've had this happen a few times but it didn't cause the wheel to seize.
The freewheel it happened with was a very very cheap no name freewheel that came with a cheap bike.
After I replaced it, it didn't happen again.
My theory is that the old freewheel wasn't spinning truly either because of it's cheapness or because the cone wasn't tight enough.