What if you have an open electrical box on the side of the house and when you turn the sprinklers on everytime it gets to a particular zone it gets water in the electrical box and trips your GFI. I think it is either this or that you may have a bad solenoid valve.
You will like my other Idea.
I think your timer is tired after watering the first 3 or 4 valves.
End of season Humor.