There is two common mechanisms of injury for meniscal tears. Usually, in younger populations, this injury occurs with acute traumatic movements, (i.e. suddenly twisting or rapidly changing direction) with a bent knee.
In older populations, it can present with gradual onset sore and stiffness or tears with simple movements (i.e. getting out of a car or kneeling to pick something up off the floor.) This is the natural age-related wear of the meniscus over time.
The image demonstrates the various kinds of meniscus tears that can occur.