Piezo circuit issues
Piezo elements can look quite inductive to the circuit.  They can also produce high voltages from external shocks.  Some barbecue lighters make a spark electrically by whacking a piezo, for example.

The problem is that either or both of these effects are causing high voltage which is frying your transistor.  Make sure there are reverse diodes to ground and power to provide a safe path for any current that would otherwise cause a high voltage.  Schottky diodes would be good in this case since they are fast and your voltages are low.