I rode my bike (Trek 720 Multitrack) to a doctor's appointment four days ago and left it outside for about 45 minutes. The temperature was approximately 20F, and there were 20-30mph wind gusts. I got back on the bike to ride to my office, and when I got up from the seat for more power on a hill, my gear shifted. I thought maybe I accidentally shifted while I was pedaling, but then it happened five more times in a mile, both standing and sitting.
The bike was in my apartment all weekend (I always keep it indoors at home and at work), and it seemed fine yesterday. But this morning the gear shifted by itself when I was about 40 feet from my office. It seems to happen 99% of the time when it's in fifth gear, and if it happens in another gear, it's usually 6th or 4th (my big gear is at 2). I had the bike fully tuned up and a tire replaced a few months ago.
I'm pretty bike-dumb and mainly just use this bike to go the mile to and from my office five days a week and maybe run a couple errands on the weekends. Any ideas as to what could be causing this? Your help is greatly appreciated.