That is detonation. Broke the ringland and excavated out a hole probably. If you don't fix the setup for the correct AFR & timing it will happen again. Look how clean the top of the piston is that didn't fail - totally lean. Look at the scuffing in the two corners on the bore. The bad one has oil burned onto it from the blowby.
Engine Basics: Detonation and Pre-Ignition