What would cause timing belt to shred and seize engine

I had the belt replaced by lithia at 123000 miles and shredded at 129000 miles and locked engine up and they told me it was my fault because the tensioner and waterpump that was replaced at the same time as the belt was not the cause.

I think there are several questions needing resolution before going to small claims. Who diagnosed the belt failure and did they cite a cause? Was the repair shop who completed the original work licensed and is there an invoice? Has the original repair facility inspected the vehicle? Also, exactly why (reason) do they not believe it is their responsibility? Some shops only have 90 day warranty periods...even then a reputable shop will address the problem. Any repair facility should provide detail of their position in a matter like this...new parts do fail and even the best Technicians can miss things. My belief is that any professional licensed facility would willingly address this problem. I really hope you will provide more detail/context.

The work was done by lithia Hyundai and was under warranty when it failed.

So what did they cite as the cause of the failure/engine siezure? If the engine seized due to an internal failure up it would have sheared teeth off the timing belt. If a timing belt component (tensioner, idler, water pump failed) it should be easily evident. Would really be interested in their explanation...are they claiming the motor failed internally and that damaged the belt?

"What we got here is a failure to communicate" .. {Cool Hand Luke} 1967