I think its glaringly obvious why he's kicked this off. Her behaviour threatened his career. He was looking at the real possibility of it being all over for him, despite a few folk who had heard his side of the story believing him and standing by him (JK Rowling take a bow). If he wins this case then it's a big step towards vindication.
If what she's said about him turns out to be true then he deserves everything that's coming to him. OTOH if she turns out to be a proven liar then I hope this ruins her - and makes anyone else who's thinking of making similar false accusations think again.