More details from Fox News:
According to a source familiar with the process, Van Susteren asked to renegotiate her contract after Roger Ailes resigned under pressure as Fox News chairman following a sexual harassment lawsuit. Van Susteren is one of a number of key Fox personnel whose contract included a “key man” clause allowing her to leave if Ailes departed.
When those negotiations deadlocked, Van Susteren chose to invoke the departure option, the sources said. Her last show was Thursday, and the swiftness of events did not allow her to deliver an on-air farewell.