There's an odd bug going around, targeting Nexus 5 devices with Android 4.4.2.
Several users have noted that when they fire up an app that utilizes the camera, the Nexus 5's processor maxes out and stays there, causing some serious battery drain.
It's unclear how many people have been affected by the bug or if it plagues other devices with non-stock versions of Android. Google has only verified this issue with the Nexus 5. In the official Android Issue Tracker forums, a Project Member writes:
I can't provide an estimate for when the maintenance update will be ready, due to all the testing we need to do for this and other fixes. Until then, rebooting the phone is the only way to stop the high CPU usage/lack of camera function once it starts.
Not sure if you're afflicted? You can head to Settings then Battery to see if "mm-qcamera-daemon" is being a "demon" to your device.
Until a maintenance release (likely Android 4.4.3) is pushed out, Google suggests uninstalling apps like Skype that actively use the camera app. "Other camera-using applications may trigger this bug as well, but that's been relatively rare," writes the Project Member. "Most applications also do not access the camera when not in the foreground, so they will only trigger issues when actively used."