It’s undervascularized. It doesn’t fail so much as the normal process of microtrauma-and-repair that strengthens muscles is mostly just microtrauma in tendons. It was once thought he damage was inflammatory, but histological studies have shown otherwise: the tendons degenerate into more-poorly-organized collagen; effectively, a disorganized scarring.

There’s a great article on it. I was going to summarize it, but honestly, I think it’s super accessible to non-medical folks: https://academic.oup.com/rheumatology/article/45/5/508/17888...