If the upper part of Pec Major that attaches onto the collar bone becomes too contracted & tight, the collar bone can’t move enough ... so the Acromioclavicular joint will move excessively to compensate. (This can happen from overworking the Pecs in gym, or as a result of scar tissue as the clavicle heals after fracture.) So, releasing any shortened upper Pec Major fibers can be a massive help in AC joint rehab.
A tight Pec Minor will pull the shoulder blade forward and this excessive forward tilt of the scapula can put torsion on the AC joint too, causing an inflamed, painful AC joint. So releasing the Pec Minor is essential too.