1. No Injury at All
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.
Many patients come in to see me after a fall. This is often simply a slip on the ice but sometimes after skiing or biking or other activities. They have a normal shoulder beforehand and now have pain and can't lift their arm up.
We often time see rotator cuff tears in overhead athletes. Athletes who spend a career throwing can easily develop partial thickness and full thickness rotator cuff tears over time. Treatment here is often time aimed at throwing mechanics and prevention.
Did you know that when you are over 40 and dislocate your shoulder you have a very good chance of tearing your rotator cuff? This is very different than the teenager who dislocates their shoulder. They typically tear their labrum (the cartilage bumper around the shoulder) and their rotator cuff is fine. The 60 year old who dislocates their shoulder will usually have a rotator cuff tear.
5. Reaching and Lifting
This is one of the most common ways to tear your rotator cuff and everyone is susceptible. When your arm is reaching out and then you lift something, the rotator cuff is under a lot of stress. For example, I see a lot of rotator cuff injuries from someone simply reaching into the back seat of their car and lifting up their briefcase. Always take the time to walk close to the object you are picking up and keep your elbows by your side to protect your rotator cuff.