Why does RA happen?

Very complex... Rheumatoid arthritis is a form of autoimmunity, the causes of which are still not completely known. It is strongly associated with the inherited tissue type major histocompatibility complex (mhc) antigen, hence family history is an important risk factor as well.
Genetics & bad luck. Ra is caused by a combination of genetic predisposition & an environmental trigger. The trigger turns on the immune system and it then turns around and starts to attack the body. Identical twins have less than a 50% chance of both getting ra. Many events have been thought to trigger ra. Smoking is at the top of the list, it also often starts after pregnancy or around menarche or menopause.