Your Android phone will automatically restart after touching it in 3 days. This is the reason

Google System April update Includes new security features. If you leave you Android phone or tablet Devices that have been locked for three consecutive days will automatically restart. You will need to re-enter your passcode to open the device.

The latest Google Play Services Update is designed to better protect your data when you are not using your device. This is the latest addition Built-in security and privacy features for Android.

Forced Android resumption is essentially a way to keep your data safe without turning off your phone.