When this professor learned that her student was not doing things the way she wanted and how they had spoken, she had to punish him. She chose to humiliate him and she chose to use her shit to do it. The mistress tied his hands so that he did not resist what she wanted to do to him. It happened the way she wanted and it was fun for her.
When this guy tried to do the kind of nonsense that he had done again, this mistress was not going to give him that chance again. She felt that he was taking advantage and for that, he had to be punished. That is why she chose to make him eat her shit. He was turned into a shit slave and he had to learn his lesson the hard way. He regretted his actions.