Mistress Tiffany wanted to humiliate her slave like he had never been humiliated before. And that is why she chose to use her muddy feet to do it. She went to the garden and waded in mud before she went back to the house and she forced her slave to lick her dirty feet. The mistress only told him to stop after she had made sure her feet were clean.
Mistress Tiffany had watched foot fetish videos on the internet and she knew that they were the ticket to her teaching her boyfriend a lesson. The problem was not small and it was not big either and she wanted an appropriate punishment for him. Which is why she used her boots to trample him and make sure he would' never piss her off again. It worked and he never did.