Google has acknowledged the Nexus 5 camera/CPU usage issues in a recent issue tracker thread. This particular issue dealt with the 'mm-qcamera-daemon' which ultimately was causing some Nexus 5 users to see unusually high CPU usage which ended up meaning excessive battery drain. Perhaps key here though, the Googler who confirmed the issue also mentioned that a fix is on the way.
Details here point toward the fix arriving as part of an upcoming maintenance update. The disappointing bit about the news was the mention about how he couldn't provide any estimate as to when the update would be available. The reasoning touched on how Google still needed to do further testing for "this and other fixes."
While Nexus 5 users wait for the maintenance update to arrive -- there was a few suggestions offered. The basic, and rather temporary solution is to simply reboot the phone. Another suggestion was to uninstall Skype as this should "substantially reduce the likelihood of this bug appearing." Other apps that use the camera can trigger this bug, but it was said that has "been relatively rare."
Bottom line here, if you are a Nexus 5 user experiencing the issue you now have some hope this will be resolved soon. In the meantime, rebooting from time to time may be your best option, and as mentioned, uninstall Skype (if you can get away without having it). As for why Skype is causing this issue and not the numerous other camera-using apps, that was described as follows;
"Most applications also do not access the camera when not in the foreground, so they will only trigger issues when actively used."
And lastly, as this issue seems to have a proper fix in the works, the issue tracker thread did mention how they still want bug reports if the following happens;
"If you are seeing high CPU use of mm-qcamera-daemon, and you have not used the camera at all, or installed an application like Skype that has a background service that accesses the camera (typically, applications that provide video chat may do this), we would like to know about it."