The nominal cause
By 
Given Tucker’s extensive history of racism this seems more like the excuse they were looking for than the reason, but anyway:
A text message sent by Tucker Carlson that set off a panic at the highest levels of Fox on the eve of its billion-dollar defamation trial showed its most popular host sharing his private, inflammatory views about violence and race.
The discovery of the message contributed to a chain of events that ultimately led to Mr. Carlson’s firing.
In the message, sent to one of his producers in the hours after violent Trump supporters stormed the Capitol on Jan. 6, 2021, Mr. Carlson described how he had recently watched a video of a group of men — Trump supporters, he said — violently attacking “an Antifa kid.”
It was “three against one, at least,” he wrote.
And then he expressed a sense of dismay that the attackers, like him, were white.
“Jumping a guy like that is dishonorable obviously,” he wrote.
“It’s not how white men fight,” he said. But he said he found himself for a moment wanting the group to kill the person he had described as the Antifa kid.
It..kind of is though?
Surprising that a guy who spent his time on television sending the message that white people are better turns out to actually think that— Adam Serwer 🍝 (@AdamSerwer) May 3, 2023