Contributing factors? My ass! The additional metal to ensure there are no such failures at all, under basic maintenance, is insignificant. Begging the question of alloy selection, heat treatment, surface modification (shot peen,, etc), let alone using "waisted" bolt designs.
Stuff like through bolts should not fail EVER! ..under responsible maintenance anyway.
this is a pet peeve of mine, I know weight costs $100 per pound, arbitrarily anyway, but it annoys me when designs fail at multiple points for want of an extra 20 grams of steel, alloy or carbon in the right place.
Then of course there is the F27 landing gear story of how cracking failures were eliminated by removing metal.