I think most of it is bad luck. Micahel Floyd and Kyle rudolph, two men with bodies well-equipped for football, spent most of their college days injured. Bad luck. Zaire was bad luck. Jaylon Smith was a hit from behind.
But if I were going to chalk it up to something else, I would be curious to see the effects of:
1) less sleep because of more studying/tighter schedules;
2) trying to maximize the weight/speed balance of atheltes by getting them up to "prototypical" position weight, instead of letting them play at a more "natural" weight. I'll explain:
We get the pick of the litter of O-linemen. Heistand picks people with the body-type to be able to handle 305 lbs. Those guys don't get hurt a lot, beyond sprained ankles and such. But I bet if we were in the business of converting diamond-in-the-rough TEs to OTs, there would be more injuries. Guys like Manti can handle his 250, but not a lot of other fast LBs can (think Grace, Councell). When you take a guy like Tranquil and have him running around at 220+ instead of 205, he's more likely to get injured because of the stress on his joints. Under this theory, Alabama has less injuries because they essentially pick first, and they choose the guys that can handle the weight (there aren't very many of them).
But once again, I think MOST, if not all, is bad luck and playing football.