Simple, he counted his vacation days and they were enough, as of Monday, to get him to his official retirement date to vest all his benefits.
He probably wanted to stay to March (to extend his full pay a couple of months), but then decided he did not want to be around when the memo hit.
That, and if he stayed in the FBI he would have had to make himself available for questioning, and answered the questions. Now that he is no longer employed, he can say pound sand, I'm taking the 5th.