What happened ? Glick fault at NYCFC ?
Unlikely, since he is apparently leaving the job to move into another role in Manchester. If he were a scapegoat, they'd just be firing him point blank, not giving him another cushy appointment.
Chances are it's one of two things: either he's asked to move back to Manchester (bear in mind that he is American, so it wouldn't be for personal reasons, only for his career) or, as spiny says, it's because he was hired to get things moving on the stadium deal and he's proved incapable of doing so.