What caused this? Water? unburnt premix in cases? ??? Crazy!!
when one cylinder gets water in it, and the other one remains running normal, the cylinder with water in it will hydro-lock, and will not continue to TDC, the other cylinder has just fired and is attempting to force the water cylinder to compress, and it will not move. all the force created by the combusting cylinder has to go somewhere. the result is usually snapping the rod and slinging it out the side/bottom of the cases.
on 440/550 engines running nitro and alcohol, I've had a cylinder get too much fuel and lock, causing the locked cylinder to push the head up and rip the studs out of the cases to where they just spin by hand, and you can grab them with a pair of pliers by the acorns and pull them completely out of the top of the engine.