Mistress Mona was bored and she was looking for something fun to do. That is why she settled on shitting. She wanted to do it on camera and then check it out later. She wanted to see how interesting it would as she did it and also when she watched it later on. So she watched all the scat vids she could find and then decided to make her own.
Lady Scarlet wanted to shit on this professor because he had put her through a lot. As a result, she had to do what she felt was what she needed to do to him. The mistress cruelly turned him into a shit slave and she fed him poop so that he understood the extent of the things he had done to her. He never forgot what she did to him and he changed.
This professor gave this mistress a lot of grief and now that she was done with her school and she had graduated, she had to go back and punish him for all that he had done. She did not let him get away with what he had done to her for all that time she was his student. The mistress turned him into a human toilet and forced him to eat her shit.