The metallurgy of the hook was altered by bad heat treating. Possibly the hook was left in the oven for an extra cycle and had its metallurgy compromised so that it became brittle which could cause it to break. The other extreme, which can happen to one hook in a thousand, is that the hook's metal becomes soft and pliable, rubbery which will straighten the hook under stress. The temper is a big part of all metal products. Too hard, the hook breaks; too soft it bends too easily. Perfect world, you want a hook that has strength, pliability, and this comes from quality control heat treating. I have occasionally run into a hook that was, like yours, screwed up.