The company is making no formal announcement. But here is what I've been able to find out. [Full Disclosure: PMC also owns Deadline.com, but all its media properties are managed separately.)
A Summit rep tells me that Mitchell was shown a final cut of the film Source Code on February 24th in NYC. His review of the movie appeared on Movieline.com on March 31st. That same day, the pic's director Duncan Jones tweeted, "Find it odd Movieline choose to complain about Jeffrey Wright smoking a pipe, something in an old draft of the script thats not in the film."
The reference was to what Mitchell had written in his review: "It's up to Jeffrey Wright, as the administrator supervising the Source Code - the machine that keeps firing Colter back, back, back to the recent past - and his eccentric brio to keep the silliness from piling up like ash from his pipe. That's how you know this film is science fiction - someone is smoking indoors in the United States - and that Wright is a martinet whose malevolence must be checked."
After the director's tweet was brought to Movieline's attention, I questioned an editor there who emailed, "We've indeed been working hard for a week to ascertain exactly what happened..." When Movieline asked for a formal explanation, Mitchell told editors that he was at the screening and that it was all a misunderstanding and that he would provide a written explanation. Now Mitchell's contract has been terminated early.
Read more at Deadline.com