Sounds like your clutch bushing needs a little oil or your idle speeds too high. Consider it a blessing someone killed it before it shot out across the yard. Sometimes they'll go several yards with no riders, sometimes they'll sit and spin. Last time I did it I punched it through the drywall in the garage, ate up a piece of carpet used as a doormat, and the concrete got some deep grooves cut in it from either the clutch or rear sprocket. Good times.