The affix ḍāc comes after the word duḥkha followed by kṛ 'to make' when the sense is that of roasting.,
The verb कृ is understood and pratilomya means 'to oppose' or \to act against another's wishes\, \to pain the heart of one's master &c\. As दुःखा करोति भृत्यः \the servant pains the master\.
Why do we say \when meaning to go against the grain\? Observe दुःखं करोति कदन्नम् ॥,