If it was a tiltbed trailer, it could have been unlatched. If either the ball or the hitch were worn....that could have caused a 'pop up' of the whole trailer. If it was loaded too far to the rear or backwards (engine to the rear).....same. I think ocedy meant he normally blocks the back wheels so the car won't roll off the trailer, but that's what happened in this case.
As for a worn ball, I never considered that they could be so bad that they'd let a hitch 'pop off' the ball but I was told that they can do just that. I had that happen recently and threw away a ball that my micrometer revealed as good. Had a new hitch put on the trailer after 20 years too......after I had a 'pop off' of the whole trailer and lost it on a bumpy bridge.
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