Why does "Power off" actually reboot? If I hold my power button, I presented with the option to "Power off" my device. I touch it, and my phone (a LG Nexus 5 running on Android 4.4 KitKat) indeed turns off. However, it suddenly turns back on. If I power off multiple times, it eventually stays off. Is this normal? Does the phone restart if it is aware of pending updates or something?
Because long pressing the power button on all most any android device(if not all) will reboot the device. It is meant to be the restart button in case on a device something goes wrong, you have the option to hard reset the device.

To correctly shut down a device you have to tap the power button and select the option to power of, or if you are in CWM select "Power off device", or I think you can shut it down using adb to.