You're using a standard triac to control a highly inductive load. Triacs are prone to commutation when used in this way, that is, when the driving voltage changes phase, the inductance of the motor continues to drive current through the triac. This results in a high dV/dT effect when the current finally stops flowing from the motor, exceeding the dV/Dt rating of the triac.
A better component for this application would be a triac specifically made to drive motors. Google for Alternistor and you should find lots of info. Alternately, you could use two SCRs in inverse parallel to control the motor. (This is basically what an alternistor is).
There are commonly available from distributors such as Mouser, Digikey and Newark.
Hth,
Dave M