This was never fully explained in the movie.
Why did Patrick rape Michèle? Was this action of Patrick in any way connected to the crimes of Michèle's father? Did he similarly rape anyone else (Michèle raised this as a possibility near the end), or was it just Michèle?
There are two explicit clues I can remember:
At one point, Michèle asks "Why?" and Patrick merely responds that "It was necessary."
Near the end of the movie, Patrick's devout Catholic wife says to Michèle that "Patrick was a good man but he had a tortured soul. ... I'm sincerely glad you could give him what he needed." (Hinting that she knew full well what had gone on. Michèle seems to be merely mildly surprised.)