I have been wondering about that myself. My best guess is that there was one "tooth" of slack in the belt between the crank and the IP even though I checked it several times. It was impossible to get the pulley/belt back onto the cam. Finally, after a lot of struggling, it just slipped on like it should but that is when the crank slipped. Took it all off, reset the crank, and it just slipped on.How did the crank slip a tooth? It could have just been on the edge of one of the sprockets and seated itself after it began to move.