I am a 51 yr old woman active. Good weight...Every now and again my knee will buckle up on me. Why?

I agree. This could be for a variety of reasons. Acl deficiency, meniscus tear (acute vs. Chronic arthitis) or weakness can cause this. If this is inhibiting your activities of daily living you should see an orthopedist.
Several causes. Knee buckling without pain can be caused by weakness in the muscles around the knee. Some simple strengthening can help. If there is associated pain it can be coming from a meniscal tear or a tear of the acl(a ligament in the knee).