I haven't seen any workup of multiple major connective tissue injury statistics in the NCAA + NFL, most players aren't probably good enough to survive more than one in their careers, but I'd imagine that Sean is somewhat of an outlier as far as incidence rates are concerned. There are known genetic connective tissue disorders, as well as structural bio-mechanical deficiencies. My orthopedic surgeon at Penn - Bartolozzi, said that there was a degree of variability in connective tissue strength that seemed to be independent of muscle strength and overall conditioning on the pro football and hockey players that he treated in Philly. He was indirectly answering my question about why I had never broken any limbs or had any major muscle tears despite tearing multiple ligaments and tendons in my ankles knee and shoulders.
Sean may be just really really statistically unlucky, or he might have some physical limitations that will occur with regularity, I'm not saying that you can prove that latter, but I don't think that you can discount it either.