I don’t get that impression. I think he’s resigning because his work is effectively completed with the submission of his final report. It’s reasonable to assume he’d need a few weeks to finish closing up his files / making sure they’re pristine for expected incoming congressional subpoenas; preparing/ packaging/ delivering evidence to the relevant offices for the referrals he developed during the course of the Russia probe; and tying up other extraneous loose ends. 

Mueller was in the private sector before Rosenstein appointed him Special Counsel, so there’s no reason to think he’d stay on as a DOJ employee once the special counsel’s office completed its work. Now that it’s complete, it was generally expected he would resign from the DOJ and return to the private sector.