Google has been wrestling with a series of strange and not too uncommon bugs with a part of the camera subsystem on the Nexus 5 called mm-qcamera-daemon. Without this component, the camera on this device won't function, but sometimes it goes wonky and drains the battery. A Googler has just marked this issue as "future release" in the AOSP issue tracker, meaning it should be fixed when Android L rolls out.
The problem users are experiencing is caused by the daemon entering a stuck state where it consumes a large number of CPU cycles. This renders the camera unusable and keeps the CPU from shifting into low power mode. The Android 4.4.3 update included a few tweaks to the camera code for the Nexus 5, but the next update will further optimize, and Google hopes, finally put this bug to bed.
This is only a bug for the Nexus 5 through AOSP, though. If you're seeing a mm-qcamera-daemon entry in the battery use menu on a non-Nexus device, that's a different code base and needs to be taken up with the maker of that phone or tablet.
[AOSP Issue Tracker – Thanks, Daryn St. Pierre]