Some known reasons for this issue are -
- Netflix app on Android 4.3
- In some cases this is known to have frozen the Android device which required a hard reset to fix.
- Enabling developer options and turning off all animations can cause the device to shut down on screen lock
- Enabling 'Settings -> Developer Options' and changing a couple of settings to disable animations can cause this bug. Switching off developer options fixes the bug. Alternatively, you can keep developer options enabled but ensure that the 'Window Animation Scale' option is not disabled/switched off.