Tall grass, probably. Generally it's not a good idea to run in tall grass too much. It puts alot of strain on everything in the drvetrain. It would be particularly hard on the clutch pads and bell and the slipper, then the stock HPI rims (the stock rims are prone to coreing out the hex sockets). Other than that, I'm kind of at a loss, never encountered this problem, only similar problems where the nut was set too loose.