What does "dV/dt" mean for TRIACs?
When the current across the triac falls under \$I_H\$, which is the holding current, the triac stops conducting. With a pure resistive load this happens at the very end of the sine wave cycle, and voltage and current are in phase.
When the load has an inductive component (e.g. a motor) then there is a lag between current and voltage. At the moment when the current drops below \$I_H\$, the voltage has already risen with the opposite polarity. Therefore when the triac turns off there is a big dV/dt on the triac - "voltage is cut off immediately". This situation can lead to self triggering the triac, and it starts to conduct uncontrolled. The remedy is to use a snubber circuit, i.e. an RC in parallel with the triac.