Rearward twisting falls on skis are perfect to tear ACLs. New innovations in binding seek to fix that, but the bindings are beat and would never keep a freestyle skier in them.
feel lucky you're a guy, ladies are more than 6X as likely to tear an ACL skiing
ACL tears are almost always caused by hyperextension or twisting of the knee. And skiers have these long sticks strapped to our feet that make both hyperextension and twisting of the knee extremely easy.
think about it, whats the joint that moves the most and takes the most impact while skiing? the knee. also, think of what happens if you catch your ski in the snow when you fall or something, your leg twists and there is a huge amount of pressure on your knee.