Was the primer punched by the firing pin?
May seem like an odd question, but it had to ignite from the primer. If it wasn't punched then I am wondering if friction or electrical impulse may have ignited the primer. Never heard of it happening but this seems odd.
The firing pin may have been stuck as mentioned, then released on it's own..
Also wondering if it may have partially cocked the hammer on the seat when put into the car and slowly pulled lose allowing the hammer to fall...
Crazy stuff, glad no one was hurt.
