An abusive partner accusing the other of cheating is very often a projection of the fact they themselves had been cheating. Since they know they would cheat, and were/are, they either assume the other person is the same way, or simply don't want to draw attention to their affair. It's an awful thing.
It can also be a sign of past trauma. I.E. they were chested on before, and are projecting the behaviors of the past onto you. Also awful, but in a much more sad way.