It's also possible the tensioner failed. Tensioner failure actually sounds a little more plausible if the teeth are intact on the belt, especially since the exhaust side got the worst of it.

 

Yes, they are very tight, because timing in modern cars is set very precisely. A bit too much slack in the belt, and your car will not run properly.

Last year, a friend of mine installed new timing belt in his high mileage (1990 or 1991) Nissan Pathfinder, with the 3.0L V6 engine (VG30DE). A week later, the passenger side cam bearing broke, causing similar damage in the passenger side engine bank. Amazingly he was still able to make it home running on 3 cylinders. His technician at the Nissan dealership told him that this is a common problem following timing belt replacement on these engines. The tired cam bearing can not handle the tension of the new belt and gives up. He said that it wouldn't have happened if he left some extra slack in the belt.