Thanks JJ. It is unticked there. As an additional clue, if the backup is aborted as soon as it starts after bootup and the computer is turned off, it won't do it again after it's turned back on. Somehow, somewhere an instruction remains after the auto shutdown is executed automatically after the scheduled backup is completed.
The shutdown instruction in scheduler is "shutdown s/ f/"
The source/event ID for detection of end of backup is "backup" and "4".
Any other ideas. Thanks for helping me out with this.