Until pitch counts and appearances are tracked from the time a pitcher first pitches no one is going to be able to provide a correct answer. My guess is it's all the pitching some kids do on travel teams (not because it's travel/because the coach and parents are ignorant) from ages 9-14 when they are growing the most and the most vulnerable to growth plate and associated tendon and muscle issues.
Travel isn't the problem. Travel just provides the avenue for abuse when combined with coaches and parents who aren't concerned with arm health. A kid can now be in a weekly pitching rotation for several months from 9u to 17u. When I was a kid we had LL. We pitched once a week for two and a half months. Typically only 11 and 12yo's pitched.
Blaming travel for arm problems would be like claiming it's silverware's fault people are fat.
All true probably. So how do you dis-encourage parents from buying into the year-round baseball that is so prevalent and so often 'hyped' (including on this site) and almost surely adds a lot of pitches onto young arms at earlier and earlier ages?