Sounds like he got his head slammed in the door, while trying to get out.
That's one of the possibilities I am wondering about.
IIRC, the not dead thug indicated
the cop swung open his door hard, it bounced back, and struck him causing him to get mad and start shooting, which, having a few years working with the public, I read as,
the cop opened his door, and Baby Huey kicked it back on him then the fight started.
