This is a group of tendons collectively called the SITS (suprasinatous, infraspinatous, teres minor and subscapularis).
These muscles are small and allow the arm bone to lift and rotate. As we hang our arms by our sides for up to 18 hours every day micro-tearing can oftentimes be observed at this group especially at the supraspinatous.
This is because this muscle (supraspinatous) runs under the acromion (part of the shoulder blade) and has a very narrow space to pass through once the arm bone is lifted.
As people age it is normal for the rotator cuff tendons to wear and degrade. Risk factors include sustaining a fall or wrenching injury.