The site reports that a source close to Fox revealed the news and that Fox, "had enough and decided to get out early." Fox's publicist would not comment on the story, only reiterating a statement Fox released to People.com, which stated that it was "her decision not to return" and that "she wishes the franchise the best."
The story that originally broke on the matter revealed that it was Paramount and Bay's decision not to renew her option on Transformers 3, but it seems she left on her own accord.
Transformers: Dark of the Moon was released June 29th, 2011 and stars Shia LaBeouf, Rosie Huntington-Whiteley, Julie White, Kevin Dunn, Josh Duhamel, Tyrese Gibson, Alan Tudyk, John Malkovich. The film is directed by Michael Bay.