This is due to the differences in anatomic configuration. We can stand easily with little stress on any ligamentous structure. Weightbearing creates a force that pushes the femur down the sloping tibial plateau, thereby moving the tibia forward. This force is the cranial tibial thrust. With every step a dog takes, stress is applied to the ligament.
Ruptures come in several varieties.
There are singular incidents which cause a sudden complete rupture with severe pain and a nonweightbearing lameness. Other ruptures occur in small increments or a little bit at a time. These are known as partial ruptures of the anterior cruciate ligament. They cause a small amount of pain and a mild lameness with poor performance. When partial ruptures proceed to complete ruptures, the transition is often gradual.