Here are a few factors , in no specific order.
1, Wear and tear on bearing surface's,hooks and joint pins [hinge pin ]
2, Excessive use of heavy loads.Incorrect rim depths.
3, Slamming it shut!
4, Lack of regular maintenance,ie.seeing that the bites are sufficient and the gun is not loose .
5, Loose friends ,allowing for excessive gape thus straining the hook.
That about covers it in general,although some guns do have there own peculiarities .