First of all, it is very unusual for White House staffers to stay 4 or 8 years. It’s just the nature of the job and the huge stress that goes with it. Second of all, it is not at all unusual for staffers to leave after a midterm election because the DC calendar largely runs on election cycles. Ask most staffers how long they plan to stay in a job and they’ll tell you “until after the next election.”
Sounds like he is leaving on the eve of the midterms. Not after.