GFI is tripping---

Hi;

I have both of my pumps plugged into a GFI outlet. I know that I should
have run 2 separate lines so that if one trips, the other will still be
pumping, but I learned this too late. In the last month the GFI has been
tripped twice. I go to the GFI switch in my garage that controls these,
flip the switch, and the pumps come back on. What could be the problem?
Fortunately, I have been home both times that this has happened.

Thanks-----