Ground fault breaker

I'm going to guess that there's a surge from the flipping of the switch. Note - I am neither an electrician nor am I an electrical engineer, but a GFI generally trips when there's a surge of some sort.

GFIs are very sensitive and they wear out. I would replace it before looking for other trouble, unless there is something obvious not noted here.

I always overlook the obvious. I'll replace the GFI.
Thanks

I'd incline to Jes above, guessing gfi to be okay. Maybe there's a big spark when the switch opens at the peak of a cycle. If bothersome, connect capacitor across switch to lessen the "surge". I'd guess about 1 uf @ 1000 v