1993 Suzuki Sidekick Timing Belt Failure

I had a mechanic replace my timing belt and tensioner just for a routine 60-90,000 maintenance tune-up. While driving about a week later my car shut down and engine failed due to the timing belt falling out of the route. I had the belt replaced and thought that all was checked and fixed, however just recently the car did the same thing and shut down in the middle of the highway. I had a new mechanic inspect the problem and this time the timing belt was snapped in half. The time before that it had been pushing foward until it melted a hole in the cover. Now it snapped in half. He aligned the tensioner, correlated the crank and cam, tightened the belt properly and made sure everything was working properly to his knowledge and I watched him do the whole thing. What could be causing the belt to spring out/fall off of the mechanism?