He did not release the air pressure by removing the air leads, which enables the brakes on the trailer to apply, before pulling the pin. No air in the trailer braking system, usually means the brakes lock on.
Once he pulled the locking mechanism on the turntable, the trailer is then able to slide backwards down the hill.
If the brakes failed to apply once the trailer was loose, then it would appear that the idiot driving the truck, had wound back the brakes on the trailer, which rendered them useless. Big insurance claim will be arriving in the mail very soon.