When your knee buckles at a very young age, it’s usually not serious. But as you get older, your knees can become weaker. So when it buckles, it’s usually not a good sign. Healthline states that the number one cause for your knee buckling is an injury. They add that many cases of knee instability are caused by injuries from high-impact activities such as running. The most common injuries are ACL tears and meniscus tears. The other common cause for your knee buckling according to Healthline is arthritis. Inflammation fills your joints and will often affect the knees. Knee buckling is a common form of osteoarthritis and rheumatoid arthritis.