I just heard of a story where a position player bumped his elbow this summer (non baseball related) and an MRI revealed a partial UCL tear. I don't think that we realize that bumps and bruises can cause furhter problems later on. One of the reasons why we didn't let son play football.
Bad luck yes, son sprains one ankle in feb and most likely threw the mechanics off. The spraining of the other ankle this summer was the icing on the bad luck cake! Combine all of that with a colder season, first complete game and the arm has to be shut down. One series of events can set off another.
I agree with FBM regarding it be a combination of bad luck and poor conditioning and maybe just being stretched to the max at one point or another in their career. On of my son's teammates drafted first round in 2005 has missed almost every season, shoulder surgery. He is in High A when he most likely should be in AAA. He was throwing close high 90's in college, most likely because he was so good in HS he threw too much. I kind of cringe when I see players so young throwing so hard, especially in HS.