The active structures that provide shoulder stability include the four rotator cuff muscles (infraspinatus, subscapularis, teres minor and supraspinatus). They act to create tension as close to the joint as possible, maintaining contact between the ball and socket. 

Together, these two systems act to create a stable shoulder joint throughout the available range of motion. When either of these two systems are compromised, the ball will begin to migrate excessively within the joint, possibly causing damage. 

When the passive structures are compromised, as in the case of a torn labrum, or torn/chronically overstretched ligaments, the ball may ride up on the lip of the socket and damage the labrum. When this occurs, the soft surface of the ball comes into contact with the hard lip of the rim, damaging the cartilage and underlying bone.

When the active system is compromised, as in the case of a torn or weak rotator cuff muscle, a similar mechanism of injury may occur as when the passive structures are not performing properly. 

The underlying cause (passive or active), extent of injury and additional pathology will determine the best course of treatment. If the injury is relatively simple (the athlete has a long history of being very mobile because they stretch a lot), then strength training usually resolves the issue. If the injury is more complex, perhaps involving a labrum tear with muscle tear, surgical intervention may be warranted. A formal evaluation by a health care provider will help determine what will best suit your particular injury.