It's a good question and one for which we still don't have an exact answer. We do know that as the shoulder cocks back for the pitch, the front structures of the shoulder are stretched over and over. This includes the capsule, a fibrous material that holds the joint together.

At the same time, the bone is actually changing as well. The angle of the upper arm bone (humerus) as it connects with the shoulder socket changes over time. This is called humeral retroversion. With increased laxity in the front and a change in the bone angle, the shoulder can turn or rotate outward farther than the other side (unless the pitcher uses both arms equally, which is unusual).

Both these changes can actually result in pinching of the soft tissue structures along the back (posterior) side of the shoulder. This is called posterior impingement syndrome.