The decline really started when she denied BMW a stake in the team and they left. They haven’t had a good engine partner since then, even during the Toyota years.Whether it's her fault or not, she was in charge during the severe decline of Williams, and the buck stops with her. She's not been willing to fall on her sword previously, so I can only guess it's been part of the buyout that she gets replaced with someone else in order to move the team forwards.
She's got a young family, so maybe she's had enough of the F1 circus, and the timing is agreeable to her too.