a few months ago we watched a video tape of a fork lift going into a trailer and the driver pulled off the door dropping the lift on the ground and it was almost fatal.the rule is you can not hook unless your door is down.its the same with unhooking.at my barn as soon as we hit the dock the dock guys pop the door so when we unhook we can be written up for it because the door was up.
Another reason I think Pre-staging is a safer act than Pre-hooking! I watched one fall to the ground two years ago.. luckily.. he held on and did not jump off...
Can happen with any carrier at any dock.. be safe.. check/double check..