TRIAC controlling a light self-triggering for very short periods
Triacs have a dV/dt specification, above which they can self trigger.

Interrupting loads on the same circuit (so lights in adjacent rooms) can easily cause spikes on the supply.

A snubber will go some way to help, it will increase the threshold, but may not be enough. As these spikes are differential mode, a common mode filter may not be enough.