Hi i run small engine technology. ..the link above. 1. Idler/clutch pully is binding under load 2. Bad bearing 3. The belt guides have moved against the belt...when it engages. ..runs into the guide. 4. Transaxle problem Loose motor mounts can do it too. The belt will usually slip when that happens Bad brakes can jam up too/parking brake can jump in if it has one ect Believe it or not if the shifter is worn or out of whack it can do that ...snowblowers do it too. They even change direction some times in mid use haha...im just speculating of course..depending on drive type these may or may not apply. Its hard to say without seeing it Im in novar if youd like to bring it down id be happy to look it over. Rate is $40/hr. I have 18 years experience in the trade and ran several rentalls. There isnt much i cant fix.