On the basis of my experience, I would say that the more physical demands made on the player in everyday play, the more likely he is to get injured. A SS or Pitcher with a 37 health rating is an injury waiting to happen, a LF or 1B can get by for season(s) without too much happening. (I'd be interested to know if other managers have noticed the same trend.)
That said, I do think way too many 99 health pitchers go down for year + injuries, even with 20 mil in training budgets. I'd be perfectly happy to see more injuries, but with fewer serious injuries, to bring things more in line with RB. After all, most injuries don't even make the 15\21 day DL in Real Baseball, but they do occur.