Check out this video.
As a surgeon who sees between 50-100 shoulder patients a week, I see lots and lots of rotator cuff tears. Interestingly, most patients have no injury at all. They simply wake up with shoulder pain that doesn't get better. In these cases we assume the tear is degenerative in nature. That is to say, the tissue of the rotator cuff (a tendon) becomes less healthy with aging and gradual wear and tear. Eventually the tendon tears completely and pain and weakness are the result.