Some rotator cuff tears come from a single injury such as a fall, a push or pull, or throwing or lifting something heavy. Most develop from repetitive strain over a long time.
A common reason is impingement. The space the rotator cuff tendon passes through becomes too crowded, and the tendon gets pinched each time you raise your arm.
People who do a lot of overhead activity are at the greatest risk, including those who play baseball, volleyball or tennis, row, lift weights, swim or do archery, and those whose work involves carpentry, painting, hanging wallpaper, cleaning windows or washing cars.