It depends on where and when you feel the popping. If it is in the back part of the shoulder blade, the popping might be coming from a problem called snapping scapula. Snapping scapula causes feelings of grating, grinding, or snapping under the back part of the shoulder blade when you move your shoulder.
Popping felt during shoulder movement can also happen when a tendon rolls over a bony bump. The acromioclavicular joint is formed where the shoulder blade meets the collarbone. This joint tends to wear out earlier than other joints and can also start to pop and click with shoulder movement.
Most cases of popping or snapping in the shoulder are not serious. They often can be helped by a well-rounded exercise program designed by a physical or occupational therapist.