Why is it turning off

If it shuts down consistently at the exact same time every time I would suspect that something like an app or some apps not configured correctly or even something is the settings is causing it. It could be many things. If you don't have anything too important I would suggest doing a hard reset using power and volume down when you boot up to enter fastboot, them volume down to select Recovery then power and volume up to get into the next screen, choose Wipe Data/Factory Reset then Delete All User Data. It will delete all your data but if you can't get past the security swipe you can't access it anyways unless you have TWRP installed. After that setup your account and take note of anything funny and what you did before something acts up. Installing TWRP recovery is a good future precaution to restore back to an image you can create at any time then when have a problem you can use the buttons to restore to that point.