There may be two types of arthritis in a cat, osteoarthritis and rheumatoid arthritis. However, the latter condition is rare and most of the times, the reasons for arthritis might be the former one in which there is degeneration of the cartilage of the joints. Some of the factors which contribute to this condition may be: 

1. Over weight: Abnormal weight can be a leading cause of osteoarthritis in felines. Weight reduction in that case can be the best treatment.

2. Mechanical stress: Sometimes some kind of trauma or mechanical stress can contribute to such kind of medical condition.

3. Severe degeneration of joints due to infective arthritis: Either by penetration of sharp objects or infectious organisms, there might be infective arthritis which may cause severe degeneration of the joint. In such a case, there might be extreme pain and inflammation. The organisms that are responsible for this may be Erysipelothrix, Staphylococci, B-hemolytic Streptococci and Hemolytic E. coli.

4. Autoimmunity: Autoimmunity triggers may lead to rheumatoid arthritis. What causes this problem is still not known. However, these triggers may lead to inflammation of synovial membranes.