The explanation I heard was that it can be blamed on the travel squads and youth baseball that is almost a year-round job now. So pitchers throw more, harder, and overwork their arms froma young age. By the time you get to be 25 now, you have the arm of 40 year old cause you have overworked it. That is the story a pitcher having Tommy John's said.
I suspect that is exactly the reason. A lot of those kids throw multiple games in a weekend too, plus play the field making long throws. That is on the coaches and parents though.