air fuel mixture ignites before the piston is ready to travel down the bore. Thus putting extreme forces on the bottom end of the motor cause the piston is trying to still go up while the explosion is trying to force it down. Hard on everything if bad enough something will give up. The cause can be a combo of things most likely to much cylinder pressure for the fuel ( octane rating). Aslo can be related to a lean air fuel ratio. This is a quick and very dirty explanation but I hope it helps.