It's called Trump lining Session's pockets with the green stuff and a promise to be one of his cabinet members; VP seemed to be the most persuasive.
Sessions might have looked at his career path and decided that McCain (Armed Services) and Grassley (Judiciary) were intending to die in office and any shot he had of being a committee Chairman was not going to materialize.
Also, he is the Chairman of the Judiciary's Immigration and National Interest Sub-Committee so he might just have the first hand experience to conclude that Trump's plan is the only one that can stem the tide.
I don't know that I've ever looked at Sessions as being particularly venal or craven. I've always liked him.