Glad you found the problem, that was going to be my first thing to check.
As for why, any little bit of moisture will trip a GFI, that's what they are designed to do.
So if you happen to splash some water near an outlet it can trip it. Happens quite often. Anytime we have a mysterious outage like that the first thing I check is the GFI. Sometimes they seem to have a mind of their own.
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2014 Raptor RP300MP w/ Rear Patio Party Deck, Folding Side Ladder, 6 Point Level Up, Carlisle Radial Trail HDs
2004 2500HD D/A CC SB - TTT Mirrors, Prodigy BC, 18K Pullrite Superglide, NEW Fuel Injectors 11/2015 (ouch)