Lady Shay told her servants that she did not want any secrets between them and everything went on well until she realized that this particular one had secrets that he did not share with her. She was so pissed and she sought to make an example out of him. The mistress slapped the crap out of him and she dominated him so that he would learn his lesson the hard way.
Lady Shay is not insecure but that does not mean she tolerates bullshit. This guy tried to body shame her so that he could take advantage of her resultant low self esteem and then nail her. The mistress could not have any of it and she rained heavy and hard slaps on him to teach him a lesson he had never been taught before. The mistress made him regret his approach.