1993 Subaru Loyale timing belts

Last year our mechanic replaced broken timing belts. Now, less than 1 year (and under 2000 miles), after the car wouldn't start and went back into the shop, he told us one of the belts "just fell off." Is this possible, with the car not even running or moving? Or has he missed something? Any precautions or futher complications I should be aware of? Thank you.