When the lights get turned off, the field collapses and it can act like an inductor, putting a spike on the line. It might be enough to couple to the other leads on the GFCI. Does it have a load side that is just capped off? Have you tried one without load terminals or pigtails (assuming you don't need them)? Would be interesting to record the info, but those test tools are pretty expensive, a typical scope is lousy at catching single events or triggering on them as well.