I have had my phone do that, some launchers take a screen shot when you press the back key and the lock button simultaneously. I think sometimes these soft keys "get stuck", causing that. I know I've had it happen to my home key too, causing the task manager to keep popping up (very annoying).
Usually mashing the problematic key a couple times clears it up and it acts normal again. Mind you, this only happens every now and then.
Oh, and I say some launchers because I have been running AOSP launcher for a decent while now.
Sent from my SCH-I500 using XDA App