I believe I may have found another reason Brett McGurk resigned. It seems he had an email problem. Popular topic theses days.
https://www.huffingtonpost.com/2012/06/15/gina-chon-email-wall-street-journal-brett-mcgurk_n_1601343.html
This is what got my attention: "She went on to clarify some details about the emails, saying that McGurk never gave her classified information and that the messages had been “misinterpreted.”