Mistress Agma and mistress Agata could not wait to torture this slave and they did it because he was slow in completing his tasks. He took his sweet time to do things and that was not how the mistresses wanted things to be done. So the mistresses farted on him so that he would be degraded enough to not want to disappoint them. The punishment worked and he improved his work rate.
Mistress Dorimills and mistress Agma wanted to try fart fetish but they did not want to dominate someone who did not deserve it. That is why they chose to pay someone to do it. That way, they would not feel guilty or bad at what they did to him. He agreed as he needed the money and the mistresses had fun facesitting him as well as farting on him with their hot and curvy asses.