All of my tendon ruptures have been the result of sudden, immediate, unexpected reactions.
As touched on prior, my muscles have become acclimated to heavy, controlled, predictable loads from decades in the gym but have never become accustomed to unseen, unpredictable reactionary loads.
Imagine a quad tendon while squatting: slow controlled resistance gradually introduced to the tendon. Now imagine slipping on ice, or falling down stairs: you just react instinctively to try and prevent loss of balance and injury. That introduces and instant full effort load on to the tendon to which it's not used to. You contract a quad instantly and the weakest link lets loose, which is the tendon.