If you have upgraded your phone from android kitkat to lollipop recently, there are chances that your Nexus 5 runs into issues like restart on its own every now & then and its a restart loop most of the times. Another issue can be, the power button gets over sensitive, even touching the button lightly switches off your device.
I had the same issues and my nexus was already out of warranty, so there was no way to get it replaced by the google. I had to go through a lot of tips, tricks, support & fixes until i finally got the issue fixed.
Before i performed the upgrade to android 5.0, i had all these applications like CM security, Clean master, True Caller installed and running fine on my android kitkat. But soon after the upgrade, my nexus started acting erratically.
Source:
http://www.coolcoder.in/2015/01/quick-fix-for-nexus-5-frequent-restart.html
