Lady Shay is the sort of person who likes to punish anyone who messes with her regardless of their status. As long as she is able to do it, she will do it. And that is why she chose to use her rubber boots to punish her boss. He had been giving her a rough time to make him sleep with her but she was not about to do it. Instead of doing so, she punished him.
Lady Shay learned that this guy had done what she had warned him about. She was so mad at him that she chose to make sure it would never happen again. The mistress did this using her rubber boots. The mistress used them to trample and to degrade him by making him lick the soles. He also had to lick her bare feet before she felt it was enough punishment.