"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."
The update bumps the OS version to 8.10.14234.375 and the firmware version to 02040.00021.15063.02004. If you're facing the issue and are yet to see an update, Microsoft has a fix.
Thanks Mohammed, Kavi Raj and Sadananda!