The metal movement during a strike forces the metal into certain areas during the strike. As the die ages and get polished, it can crack. It seems that the single squeeze dies that do not have a much of a profile to the design, then to crack a lot earlier than the multi-hub dies of the past. The memorial reverses had a tendency to crack on the 4 corners of the dies back then. Now the shield dies have an issue with the upper left quadrant of these dies.