I have maintained for years that many of them have been threatened, including their families. They were told that they could make noise but not cross the line. Finally, under DJT, some are finally crossing the line.
Trump was/is wise not to fire him....yet. The Dems have wanted Trump to fire him, especially earlier in the Mueller “investigation”.
Sessions could be replaced with a recess appointment. Joe DiGenova would be my choice. The Dems would only vote to confirm a Barney Fife type who would be worse than Sessions.
Trump is fighting them with the resources he has and is being smart about it.