I think I'm having the same problem. Sometime after 10 pm yesterday til noon today, my phone turn itself off. With very light usage too. I did check "Fast Boot" like a day ago or so. I have not downloaded any new apps or updates. Last download was Chrome. I don't have a lot of apps in general installed. This is a stock LTEvo. No wifi on.
Check the attached photos.
You can see the battery just took a nose dive last night.
Today I hit the power button and nothing happened.
I held it for 2 secs and it booted from from fast boot with 48% battery left.
I have not charged my phone for days.
This is my first incident.