Vagas is right, any sort of plating or especially galvanizing will effect the strength of the metal, except for maybe paint and powder coating, altho the powder coating is baked on right? So that might anneal the metal slightly... More than likely it is metal fatigue. If that vertical piece isn't wide enough, it won't support the weight of the chain guard as it bounces with each bump you hit, and bends slightly. Think of it like gently bending a paperclip over and over till it cracks.
