I see that you have the problem fixed, but just in case, here's another thought:
I have several old Cub Cadet garden tractors that turn a belt 90 degrees to move power from the horizontal engine shaft to vertical driven pulley on the mower deck. Most of the time when a belt comes off it's because the belt itself is bad. Sometimes I have to look closely to find the defect.
I have learned over the years that all vee belts are not created equal, by a long shot. Especially when subjected to this kind of twisting. Some of them take a permanent or semi-permanent set when sitting for a long time. Not a problem in "normal" service, but not good here.

(There ARE other belt-retention problems with these old Cub Cadets, which I detail at:
http://www.spaco.org/CubCadet/MowerBeltComeOff.htm


Pete Stanaitis
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