Shoulder popping can refer to two distinct phenomena: noises made by the joint or dislocation of the joint. Shoulder popping in terms of noise is due to wear on the joint and is generally benign, whereas dislocation is caused by excessive force applied to the top of the arm.
Noisy shoulders are a relatively common occurrence and generally do not need to be heeded as a health problem. The shoulder undergoes a lot of use over the course of everyday life, and aging naturally causes the joint to roughen. The shoulder contains many disparate kinds of tissue that sustain different degrees of wear, and if one or many of these are not smooth, they cause noise when they rub together as the joint moves.
Noisy shoulders can, however, sometimes be cause for concern. If a shoulder begins to click after receiving a blow, it may indicate serious injury to the cartilage, especially if the joint is painful or feels unusually loose.
Shoulder popping in terms of dislocation is brought about by sharp force applied to the top of the humerus, the bone in the upper arm. The humerus is coupled into the shoulder blade by a shallow cup-and-ball joint, so popping it out of place is easier than the corresponding injury to the leg. Most cases of shoulder dislocation are caused by a blow to the front, though it is also possible to pop the joint by striking it from behind. When people fall, they often stretch their arms toward the ground to protect the head or torso; this stretching is often the cause of shoulder dislocations in the elderly.