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It's about the way metal is worked. To make a hook keep it's shape it needs to have a certain amount of "work hardening" or temper in it. In many cases the material is worked to a point during bending and forging probably gets the material close to it's ultimate strength. After you cut the hook, open it and close it again you are very close to going beyond the ultimate strength and the hook is subject to failure. Add to that the protective coating may be damaged in the process and you have corrosion sites that can further weaken the hook.
One bend you are OK, like Mike P says try to adjust it after it is closed and you take the metal beyond what it can handle and the eye fails. Smae thing with the SS siwash hooks.
After all that I still use open eyes......
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