Though his review on that specific game does seem like the most likely cause, loads of people on the GameFAQs boards have been ragging on his reviews lately, especially on Nintendo boards. Since the 2 sites are linked, the complaints may have played another factor.
But both of these things shouldn't even be factors by any means. Reviews are about stating your opinion on something, and not appealing to a certain crowd, or advertising for someone. His being fired should be under entirely different circumstances.