The power turns off unexpectedly. Why?

The Power Management function may be running.

When [POWER MANAGEMENT] is set to [ON], the unit automatically enters Off mode after 20 minutes of inactivity if no audio signal is input to the unit. The [MASTER INSERT/(WAKE UP)] button lights up in Off mode. Press the [MASTER INSERT/(WAKE UP)] button to exit Off mode.

For details including how to turn off the Power Management function, see [Settings] > [UTILITY settings] > [Power Management] in the Instruction Manual.

The Instruction Manual is available [here].