I'm guessing David Humphreys is moving into a coaching/selectorial role with Gloucester and as such the move helps him develop his skill set in another facet of rugby. With Ulster he was, to borrow an American sports term, a General Manager, responsible for providing the head coach with the best possible team. As Gloucester he'll have a more hands on role over what happens on the pitch. It's hard to begrudge him the move even if it's a hell of a shock and loss to Irish rugby.
Who are possible candidates to take over? Brian McLoughlin, Allen Clarke and Gary Longwell could be "in house" contenders.