i've torn my acl twice playing football. as cobri66 said, an imbalance of quad to hamstring can lead a greater chance of tearing it. heard that from Dr. Albright at UIHC and the other orthopedic surgeon who fixed my knee.
for some people, it's a freak accident. for me, i got it blown out willis mcgahee sytle. i am sure some people, due to their knee structure, biochemistry, etc are more likely to tear it. seems like running backs sure put a lot more stress on the ligament due to making cuts, stopping.
we have seemed to have quite a few - but to be honest, don't follow other programs as close.
also - in line with your thinking - since we get the under-rated/ blue collar types, perhaps they naturally have to work harder to put on the muscle, they don't have the god given frame of some other schools? some people are supposed to be 250 lbs, some are not.