Maybe at FOX she was ordered to read script she didn't approve. When she started to balk, poof! she's gone....
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...And maybe she just got "too big for her britches" ! recall that questioning period during aback, when he was immediately attacked by her during the 2016 presidential campaign.
After being treated totally unfairly by all the other major media, think Trump expected to get a fair shake from Fox at least. I am sure he was completely taken aback when he was immediately attacked by Megyn.
When she did attack him, it was all on a single issue which she apparently felt very strongly about. Being on Fox News, she had to know that Obama had really driven the country into the dirt, and 4 years of Hillary, would likely destroy it!
Still, she had to attack over her petty butt hurt, no matter that she may aid in destroying the country!
Nevertheless, Trump took her apart during the exchange, and she likely helped rally support for him from among those who are tired of 'political correctness' ! (2+ min video of the exchange)
https://www.youtube.com/watch?v=1Y9_LJj7A68