Between Trump's tweet calling him out, Barr's visit, and his sudden departure, it's easy to connect the dots and think he was pressured out. However, I think he was finally just fed up with working for a news organization that functions as a propaganda outlet for Trump.
Maybe Barr's meeting was the final straw, even if he wasn't aware of what the conversation was about.
There's long been tension between Fox's news division and their opinion hosts and that line just keeps blurring. And frankly, while he and Chris Wallace have done good work there, their presence on the network have mostly functioned to provide a veneer of credibility to a network that doesn't deserve one.