I think it is from straining to jump up on the stairs or whatever. If if happened to your 12 yr old dog, it could happen to your 5 yr old, too. They all age at different times. Especially if they don't even know it's happening. I would just make it easier for them so they don't have to jump, i.e. stairs, ramps, etc. One of my girls has that luxating patella thing, and I am going to get her a ramp to get up on the bed, and one for the couch, too. Good luck.