What is the cause of synovitis in neck?

Joint inflammation. There are several cervical facets or neck joints. In fact there are 2 at each level with 7 levels giving you a total of 14 joints. Over time these joints can "wear out" leading to degeneration and inflammation of the joint. When a joint becomes inflammed it often swells due to increased synovial fluid. The term for an inflammed joint with excess fluid is synovitis.