I didn’t think there’s a specific disorder that results in accent changes. It could be a simple as wanting attention - maybe wanting someone to ask her about it - or it could be she has some psychological disorder.
There is a thing called “Foreign Accent Syndrome,” but as I understand it, it just affects the way your mouth and tongue work in a way that others perceive it as a foreign accent. I don’t think it would cause shifting from one accent to another.