Sorry mate, I missed the question. Wasn't meaning to be rude.
Basically all steel things have a fatigue load limit. Below that load you can load infinitely (faults notwithstanding) and it won't fail, but increase the load, or in this case torque, and eventually it will start to crack slightly more and more each load cycle until the cross sectional area holding it is too small and just lets go, like the coathanger.