When I pulled a toad... All tow bar connector pins used clevis pins through the pin to prevent loosening. Never had an issue.
mikeh,I take it that you ruled out tampering?
I use the NSA elite ll. It comes with pins that have a large D handle at the end for easier removable. But stock came with hairpin type keepers. When towing the pins tend to work out so that the keeper pin is tight against the vehicle bars. Itís tight with the safety cables very close and has to be doing some rubbing in tight turns. Just wasnít comfortable with that and now use pad locks to secure the pin. The hardened padlock is much more secure than the flimsy 1/8Ē safety pins. I carry spares pins from tractor supply.