You most likely trigger "Quick turn on camera" action. It launches camera app when power button is pressed twice. It's located at Settings > Buttons & gestures. Either your power button is defective or you press it twice by accident.
Pro:
Fast and simple
You can see the image you have just photograph not access to other area unless you unlocked your phone
Con:
You can not access to previous image
You can't access to other areas it is locked
You won't be able to access any app unless you have unlocked your phone or anyone can't access also
