The majority of ACL injuries to skiers are caused by what's known as "phantom foot" or, in plainer terms, a backwards twisting fall. Basically backslapping and catching an edge at the same time.
To put your tails down first, bearing in mind that we're landing on a pretty much flat surface, means you would have to either flex your boots (which you can't as there's nothing to flex them against) or straighten your legs and draw your thighs up. This will isolate the load onto your quads and result in some sweet backslap action and a higher chance of popping your ACL. If you fall forwards you have virtually no chance of doing an ACL.