These mistresses realized that this guy had a thing for them and they wondered how they could take advantage of it and have fun at his expense. They went out of their way to tease him and entice him. And since he had a thing for them, it was easy for him to take the bait. And that is how they managed to ride on him as well as crush and trample him for fun.
Mistress Cucciolo was angered by her girlfriend because she had gone against what they had agreed on. So she felt that the best thing to do to her was to turn her into a human pony for fun and torture her like never before. And that is exactly what she did to her and made her feel pain and be humiliated as well. She learned her lesson the hard way.