 Some of the fibers on the inside are starting to pull apart and it starts to fray. As the tendon gets thinner and thinner though over time, it gets easier to tear. So that's why people with maybe not major traumas in their 50s and 60s, they pick something up and their rotator cuff tears because the tendon is getting thin to begin with. My favorite is people that put their luggage in the overhead bin. They feel a little twinge. They keep hurting, keep hurting, and then they get an MRI and it shows that they've torn three tendons.