The shoulder joint is a ball-and-socket joint where the head of the humerus (upper arm bone) fits into a shallow socket called the glenoid, part of the scapula (shoulder blade). T The socket is relatively shallow compared to other joints, like the hip, making it less stable. Shoulder dislocations often occur due to trauma or impact, such as falling on an outstretched arm, direct blows to the shoulder, or forceful pulling or twisting motions. Sports injuries, accidents, and falls are common causes.