If you have a Nokia Lumia 930 you may want to check for an update. There is a critical update coming to stop it from crashing and spontaneously reboot. While Microsoft didn’t state what causes they apparently know how to fix it.
They have stated:
“We identified a problem in some versions of Windows Phone 8.1 that could cause a phone to restart periodically. A critical update will fix this problem. If you’re unable to install the update or want to learn more, we can help you fix your phone.”
In the update itself reads:
“An important update is ready to be downloaded. Windows Phone 8.1 – This update fixes a critical issue that may cause your device to periodically reboot. Please install it now so that your phone will keep working as intended.”
Check for it to see if it’s available for your device and let us know it if it helps.