This sounds alot like mine. only happened twice, but thats been over the last week
Basically, when i was scrolling through the gallery there was like an electric buzz coming from the back of the phone, i could even feel it vibrate the phone when it did it, the phone then locked up and i had to restart it (holding the power button down).
Today i took a pic and pinched to zoom to look at the pic and the same thing happened electric buzzing and the phone vibrating from the back, the phone didnt lock up but it rebooted anyway :/
Iam on stock 4.4.2, never rooted or anything like that