I suspect he was in a hurry and thought he had put it in park.
It says he fell out. I’m guessing he was leaning way out of the open door to try to see his trailer behind him. Leaned too far....
I watch the cop chases on youtube every-so-often. It is amazing how many times the cop finally gets to the pursued car (often ends in a crash) and the cop jumps out of his car to give chase or get a gun on the suspect, only to have to go back and put his car into park as it starts to roll!