Ive never had a problem my GFCIs tripping in the rain, and it typically rains about half the time where I live. Maybe the rain where I live is distilled, so it doesn't conduct electricity.
...Wait, even though I said that to be funny, would it be possible that some regions have acid rain, and therefore conductive stuff in the water?
Also, is this something to do with the lights being on controllers, as I've never heard of anybody having problems with a static display, either. Maybe is it more prone on incandescents vs. LED strings because of the higher power draw? Or strings with removable (led OR incandescent) bulbs vs. non-removable ones?
Perhaps some of the people running into problems with tripping GFCIs chime in on their type of lights and if they live in s "boogey man" acid rain zone?