As others note, that is the classic symptom of the overcenter spring
being broken on one side. The spring in question is a two-sided
torsion spring with anchors on both sides of the overcenter arm. The
result of this failure is that the hook will seem to operate normally,
but it will self-release under very low loads.

It could also be the other failures suggested in this thread, but my
money is on the spring being broken on one side.

Thanks, Bob K.