Glenn Fleisig at ASMI says that they do not have any evidence that mechanics of any particular sort actually causes injuries. As I said in another thread, I believe that the M arm action is riskier but I have no proof to back that up. I propose that it promotes a later and more violent external rotation which just might be riskier. Again, no evidence.
What I feel more strongly about, with no hard science yet again, is that pitching in and of itself is a very high risk activity because the thing that causes increased velocity is also that which increases injury risk. That would be the rapid stretching of muscles and connective tissues and the subsequent elastic rebound in the tendons and ligaments and the stretch shortening cycle in the muscles where a powerful concentric contraction happens immediately after a rapid eccentric one. So, I theorize that this stresses those tissues to points near their physical limits. It is the source of velocity but we're playing Russian roulette with their abilities to withstand over time.
My amateur theories only.