A sprain of the rotator cuff or shoulder capsule occurs when there is a forceful injury to the ligaments that stabilize the shoulder joint (glenohumeral joint). The stretching or partial tearing of these ligaments causes the shoulder joint to become loose (lax) and can lead to complete disruption or dislocation.
The glenohumeral joint is the most freely moving joint in the body. Two bones come together at the shoulder: the ball, or humeral head, at the upper end of the arm, and the socket, which is part of the shoulder blade (glenoid of the scapula). Ligaments cross between these two bones and stabilize the glenohumeral joint.
Sprains of the shoulder capsule are classified according to the amount of damage to the ligaments and any resulting laxity of the joint. Mild stretching without tearing of the ligaments is a grade I or first-degree sprain. There may be some minimal instability of the joint at this point. In a grade II or second-degree sprain, some fibers of the capsule tear causing the head of the humerus to slip and almost dislocate (subluxation), creating a feeling of instability.
This from just swinging a bat? I don't think so...