This occurs when the rotator cuff tendons or shoulder muscles are stretched or weakened from misuse, allowing the unstable joint to slide forward. Imbalance can often result from overhead arm motions that are common in many sports, such as throwing in baseball.
Rotator cuff tear:
Over time, the tendon tissue breaks down. Eventually, the tendon may tear away from its attachment to the humerus bone causing pain in the shoulder that worsens when the arm is lifted.
Sometimes, a grinding or popping sound is heard when the arm is moved.