I find NASM’s “Essentials of Corrective Exercise Training” very helpful.
Under the topic “Subacromial Impingement Syndrome” (SAIS) they state:
“Rotator cuff weakness and shoulder instability results in excessive superior and anterior translation and inadequate external rotation of the humeral head, limiting clearance of the greater tuberosity under the acromion process. Decreases in the normal scapular upward rotation and external rotation of the humerus combined with posterior tilting on the thorax cause a decrease in the physiologic space under the coracoacromial arch.”
This is found on page 322 of their text book.
The sources they utilizes are “Clinical Biomechanics” published in England, the Journal of Orthopedics and Sports Physical Therapy, Physical Therapy, The Journal of Magnetic Resonance Imaging and Archives of Physical Medicine Rehabilitation.
Having quoted the above, as the posterior fibers of the shoulder girdle are involved in external rotation and range of motion is limited as a consequence of the impingement, it only makes sense to stretch the posterior capsule of the shoulder girdle.