It's a cartoon cliché. The character walks to the edge of a cliff. A crack forms on the thin protruding section of cliff upon which the character stands, causing him to plummet towards the ground while still standing on a small patch of broken cliff. But sometimes the character steps back onto the safety of the main body of the cliff, just as the small patch of cliff begins to fall. Only the small patch of cliff doesn't begin to fall...the crack that forms makes the main part of the cliff fall rather than the small bit at the end.