Pool lights = humidity = water condensation = wet insulation
Quick check to isolate the problem, does it still happen when the four circuits feeding the lights are turned off but you still operate the lighting contactor?
If so, I would suspect the inductive kick generated by the lighting contactor is punching thru insulation and find its way to ground. Is there any sort of RC snubber across the contactor coil to reduce the magnitude of the inductive kick? If not, try adding one to see if that cures the problem.
If it does not happen when the lights are off, try the individual lighting circuits one at a time to isolate the trouble making circuit.
