Most ACL injuries in skiing (over 70%) occur when the skier is in the "back seat" and catches an inside edge. The actual mechanics of the injury are simple. When your hips and knees are bent, you can rotate your leg inward (i.e. while sitting, you can place your right foot on your left knee) but you cannot rotate it outwardly. To experience this - sit in a chair (hips and knees bent), and pull your foot directly sideways to the outside. Your leg simply can't bend that way.
Skiers routinely find themselves in such a "sitting" position - pounding through bumps, landing a jump on the tails, recovering from a skid, or simply being momentarily off-balance to the rear. Flexing your ankles, knees, and hips is the natural way for your body to manage these forces, and by itself, this does not cause injuries. However, if a skier happens to be in this configuration at the moment some other force pushes the foot directly sideways to the outside, an injury is likely.
It is usually the snow that pulls or pushes the ski. If the force acts on the ski tip OR the tail, the ski will rotate around the skier's boot heel, and the toe is likely to release from the twisting force. However, if the force is at both the tail AND the tip, or if the force enters the ski at or near the heel (pushing the ski directly sideways), the forces do not twist the ski around the boot heel and ordinary alpine ski bindings will not release. Skiers all know what it's like to "catch an inside edge." If this happens when the knees and hips are bent and the lower leg is pulled too far, a knee injury is likely.