Usually the ligament is injured without contact with another player, eg running and cutting to the same side, landing on one foot while the body is still twisting (non-contact injury), or in situations where -
The ACL may also be injured when cutting to the same side as in this image.
In this YouTube video, Michael Owen of the UK suffers a rupture of his cruciate ligament (2006) as his right knee contacts the ball. The angle of impact has caused a twisting movement of his lower leg.
Less frequently, the ACL is injured in a contact injury. Here you can see two types of contact injury - both of which can injure the ACL. On the left the knee is forcibly hyper-extended, and on the right the knee is forcibly hyper-flexed.
Watch this tackle on YouTube, and the man who rolls head-over-heels (red shirt).