This is still really cool to think about, because I never really thought of an animal actively seeking out another as prey not only as food but also for other properties like chemicals that that animal may have ingested. In this situation is seems like an edge case if it ever did happen, or just as you said a happy surprise. Though that does make me wonder if there are any other instances of this in other animals and also how often it might be un-recognized by researchers. Lots of food for thought there.
As other people have said it’s somewhat all of the above, though the biggest and most visible on xray is the narrowing of the joint space from deterioration of the cartilage. This causes arthritis, and also calcification of the joint which is also all worsened with age and varying bone density which can change based on things like ancestry, gender, habits (drug use/smoking) or medical conditions.
Edit: I also forgot to mention that occupation can make a big difference here. Being overweight AND having a job that is strenuous on the knees (construction/manual labor/heavy lifting) is a bad combo which can lead to joint deterioration and premature joint replacement as well.