The Grain Mouth King pours grain from his mouth for you and his other subjects. You go to him and pull his finger. When you do this, grain pours from his mouth. But we were greedy. We didn't understand the extent to which we were harvesting his mouth wheat.
The extent to which we were harvesting grain from his mouth was too great. The Grain Mouth King has run out of grain. He worries he will lose power if he has no grain to offer, so he switches to barfing out tater (てい) tots instead.