Throwing too much, too young, too hard.
There is a 14 year old in Wisconsin throwing 87 mph. My thought is if my 14 year old threw 87 I'd never let him pitch. Young ligaments aren't built to withstand that torque.
Fortunately the Dad does limit his innings, but still. It is a marathon, not a sprint. Teenagers are sprinting towards being a draft pick and getting millions in a signing bonus. I get it. But it doesn't lend itself to career longevity and leaves many by the wayside along the way.