He's a traitor to the cause.
It is possible that he realized he wasn't going to be selected as VP and was vulnerable to overtures coming from the #nevertrump crowd.
Whatever happened is now irrelevant. Newt is dead meat.
A VP choice should be someone who fills in the weaknesses of the candidate with their strengths. Trump is ALWAYS going to be someone who tells it likes it is, the warrior against political correctness. Newt's too self-assured to not put his personal views above working in a backup position.
Who speaks more carefully and has experiences, wisdom, and accomplishments in DC and beyond? I can't think of anyone except Jeff Sessions.