AC induction motor controlled by TWO TRIACS. They blow down
Can you add more details about the PWM driving? min /max duty cycle and frequency?
Also:
-disclaimer no experience with triacs but many experience with power mosfets-
Did you measure the triacs after failure? With mosfets, overvoltage failure usually shorts gate to source, while thermal failure only shorts drain-source.

Possible overvoltage causes are caused by inductance as you imagined, thermal problems can be caused by ringing.

The advantage with fets is that you can play with gate resistance (hence dV/dt of output) which you can't do with triacs. But you could try snubbers to see if it makes a difference.