Most people have a very anterior Delt dominate movement. This is why there tends to be a lot of irritated anterior Delt and biceps tendon soreness on palpation. The Delt should be a fully developed tri head, pennate (fiber orientation) muscle. This pennate allows for force production as well as force absorption. It absorbs the rotation of the thoracic spine while walking so that the cervical spine (neck) can maintain the posture with less effort.
Often the posterior Delt is so weak that it can't do much in terms of helping centrate the joint so we get a forward translated humerous.
This exercise that I've posted on instagram will help bring up the strength of the posterior upper back.