What caused this damage

I am not going to venture as to what actually happened when the rod broke, but I will mention a couple of things. The stock rods are not a good rod for a blower engine as the fuel explosion lasts longer and puts more twisting pressure on the rods. The heavy pistons should be a good choice as long as you are not going to rev the engine above 6000 rpm. The theory is that the heavy pistons help take up the shock of the fuel explosion. By all means get a good set of H-beam rods and good bolts.

Jack