I knew they'd be done ever since reading this:
http://nymag.com/daily/entertainment...e_are_pos.html
Anyway, regardless of why, this is a good move for her. He's a great producer to use when you're young and establishing yourself. But if you keep using him you'll get stuck in a rut of big hit singles on US radio and nothing else (see Femme Fatale).
He's been good to her (CG, H&C, CG, TD, ET, LFN), but time to move on. She doesn't need to use him anymore.
I'm sure she'll keep working with hitmakers though.