Ok - this has happened to me too many times and I'm sure I'm not the only one.
You're in the middle of an unbelievable game. You're blowing up the machine. Perhaps you're about to reach the wizard mode that you've never reached before. You're in the zone and you're unstoppable.
You glance down at the time and...oh shit...you have to leave! But you're only on ball two. If you don't leave right now, you'll be in trouble with the wife, you'll miss the train...something bad. You have but no choice to let it drain and turn the game off!
Why do the pinball gods know when you have to leave and time it this way?