What could cause early failure of replaced timing belt?

My 2003 Mitsubishi Montero had the timing belt and all associated parts replaced at 82K at a local mechanic. A few days ago, at 99K miles, the car stopped running while I was driving and had to be towed (the actual breakdown had some rather dramatic metal on metal noises coming from the car). Towed car to another mechanic (we felt the other was overcharged and never returned). Well, the timing belt with 17K miles on it broke and the engine is toast. The cost to replace the engine exceeds the value of the car and I am left with nothing. The mechanic confirmed that the timing belt was definitely replaced / the work had been done. What on earth could cause such a timing belt to fail so prematurely (and sadly, 5000 miles after the part warranty).