It is not uncommon for pitchers to have hyper mobility. Doing Blackburns my son can almost touch his wrists together behind his back. As he has grown, played more and added some strength this range of motion has decreased a little bit, that is a good thing in his case.
Roger is 100% correct....developing good stability is a key injury prevention thing for guys like this. Eric Cressey has some really good stuff about this on his site and or You Tube.