Why did it happen? We simply don't know. Nor did his team doctor nor the one who did the surgery. They told him that every single pitch you throw a pitcher comes within 10% of this injury. Stands to reason that as time goes on and you throw 1 more pitch, your risk increases until 'the number comes up.' But who knows? I don't.
That's interesting. But it also provides a good reason that there is concern with the excess that has become of youth sports. All sports, with many people to blame. Fatigue leads to overuse injuries which lead to "the big one." So many "major" injuries like UCL rupture or ACL rupture are the result of more minor injuries, whether we knew it at the time or not... And so it makes sense that the more you're out there, the greater the chance you will suffer that big injury.