Just to clarify, my knee injury this past year was from biking, not skiing. I had ACL reconstruction in late fall, so it prevented me from skiing until March.
And as far as the circumstances of my injury go... I had been weight training extensively for about 8 months and was in the best shape of my life at the time of my injury. I was racing DH (MTB), put a foot down funny in some sand and POP. ACL gone. My doctor said that with the way my joints were, the intracondlyear notch was rubbing on my ACL so it was probably largely due to wear and tear over the years and that's why it failed. He drilled out the notch when he reconstructed it and in the photos you can now see that the new ACL has a nice path that it didn't have before.
Anyway - the only other contributing factor that I can think of is that I did a good leg workout before heading to CA for the race. My legs were still a little sore and that could have affected it. But overall, with this injury I didn't feel like there was much I could have done. Though now that I'm back to riding/racing DH, I'll be wearing dual knee braces (Defiances). I know it's not a guarantee, but it can't hurt. I do a lot of XC riding and other exercise without them, so I'm only wearing them for the activity where I'm most likely to be crashing violently...
With the ACL on my other knee (6 years ago), I tore it in classic skiing fashion. Early season, 4 days of bell-to-bell, got up and started skiing bumps when I was too sore to even walk normally, and I caught an edge, causing the classic rear-twisting fall. Nothing but stupidity there, but I'd never been injured and didn't know any better.