The ACL is protected in part by the amount of joint stiffness that muscle contraction provides. The stiffer the knee, the less force is placed on the ligaments. If the sudden force of an impact is greater than the strength of the muscles holding and protecting the joint, ligaments will probably be injured.
Sometimes the joint isn't ready for the impact. For example, if the player jumps up but gets knocked off center, the player may not come down straight. The angled force of the landing can cause ligament injury in the knee.