Rheumatoid Arthritis (RA) is a chronic severe inflammatory disease believed to affect around 1% of the global population. It primarily affects joints, typically resulting in warm, swollen, and painful joints. Pain and stiffness often worsen following rest. Most commonly, the wrist and hands are involved, with the same joints typically involved on both sides of the body. The disease may also affect other parts of the body, sometimes involving low red blood cell count, inflammation around the lungs, and inflammation around the heart.
“Scientists believe that rheumatoid arthritis may result from the interaction of many factors such as genetics, hormones, and the environment. Although rheumatoid arthritis sometimes runs in families, the actual cause of rheumatoid arthritis is still unknown.
Research suggests that a person's genetic makeup is an important part of the picture, but not the whole story. Some evidence shows that infectious agents, such as viruses and bacteria, may trigger rheumatoid arthritis in people with an inherited tendency to develop the disease. However, a specific agent or agents are not yet known.”
source: US National Institutes of Health
There is no evidence that physical and emotional effects or stress could be a trigger for the disease. The many negative findings suggest that either the trigger varies, or that it might, in fact, be a chance event inherent with the immune response.
Note that one of the frequently used treatment drugs Sodium_aurothiomalate, a gold salt, works by an as yet unexplained mechanism (though it's known that it inhibits the synthesis of prostaglandins and has an immunosuppressant effect).