Is there an underlying cause to numerous meniscus tears over time? i'm a healthy, active 32 year old female who has had multiple tears over 14 years

No specific cause: There is not likely a specific underlying cause. Once a knee has an injury, it is no longer a stable as it once was and has a higher risk of re-injury. As you injury it multiple times that re-injury risk goes up. At times genetics can play a role as can being involved in the same activities that caused the injury in the first place. Furthermore, body mechanics can be a factor.