Have to figure her usefulness has come to an end. She obviously stayed on at the FBI (as did Strozck, Baker, and a few others) because she was a cooperating witness with the Inspector General, along with the others. The OIG cannot compel former employees to testify to him, only current ones. So they cut a deal: stay on in a diminished capacity whereby they cannot do more damage, and they can keep pensions and reduced sentences. So now that the OIG has basically wrapped up his stuff, she’s no longer needed. Or, she felt like she betrayed Comey, McCabe, et.al. - was ashamed to stay but needed to save her hide - and her resignation is a worn out, too late protest to stand with her men (if Comey & McCabe can be so described). But it’s clear her departure means he utility has now expired. So to speak.