I opened it up and don't see any obvious corrosion or frayed wires. I'm going to do a more thorough exam of it today and see what I can find. The sockets both look good, I replaced them about a year ago because the original ones had corroded.

I woudl assume since it is tripping the GFCI it would be a short, but what are the odds of a short showing up on both ballasts separately within a week of each other?