A little late to the party, but I'll throw this out there anyway.
What causes this is the back button gets "stuck". The phone thinks you are pressing back when you're not. The key combo to take a screen shot(on purpose) is "back+power". What works for me when this happens is to press and release the back button (this seems to reset the state of the button), then you're able to press power to lock the phone (with out the annoying screen capture).
At any given time, one, some, or all of the capacitive buttons have gotten stuck like this on my phone with very strange and frustrating results. Press and hold home brings up task manager. Press and hold search to initiate voice search. These pop up for no reason when I'm doing other things due to this weird button issue. But pressing and releasing the rouge button(s) usually clears it up.