I would have guessed that it lost oil and seized in the cylinder, and just ripped the wristpin out, but the piston slid right out of the cylinder and had a nice coat of oil still on it after 3-4 weeks of sitting there.
There was surprisingly little scarring of the cylinder. But when the pin came out, it grabbed the bottom of the cylinder sleeve and ripped a chunk out of it, then pieces went flying all over the place.