Craftsman YT4000 - Drive Belt Comes off in Reverse

Craftsman YT4000 917.288570. Drive belt popped off. It was the original belt, 10 or 12 years old. Purchased a new belt. Works fine when moving forward, but when I go to reverse, there's a bit of a "groaning" sound, and in just a few seconds the belts pops off. Each time, it comes off of the transmission pulley.