This could be a medical issue such as muscle twitches in the back leg. The cat doesn't know what is actually happening so she gets angry and her tail shows the anger. Check for an injury on the back paw such as a split or cut of some type. Also, check the tail, especially the area near the rump. If she isn't fixed, her hormones could be causing the problem. Or it could be a chemical problem. I would suggest you record the action and then take your cat and the video to your vet for more information.