Menisci can be the main focus of damage during a knee injury ('primary') or it can be a consequence of another problem inside the knee ('secondary').
A tear can also be part of a compound injury when other structures are damaged at the same time.
In a primary tear of the meniscus, there is usually a twisting force with significant torque. A skier with tight bindings may have the full rotatory torque of the ski applied to this small structure. A footballer kicking a ball at an angle will have his full weight transmitted through the one knee via the meniscus.
Where the meniscus is torn in association with other injuries, the injury may not be simply due to torque. Anything which allows the femur (thighbone) to slip abnormally forward or backward in relation to the shinbone (tibia) may cause some of the forces to be transmitted to the meniscus and result in a meniscus tear. Injuries may include cruciate tears, cruciate and collateral tears, dislocation of the knee, etcetera.
Degenerate tears occur in association with arthritic disorders where there are destructive chemical substances being released into the joint cavity. The joint surfaces and the menisci undergo destructive alteration and start to break down and fragment. There is usually no directly associated injury.