Suppose it is a reaction to a viral infection you get when you are young which reactivates when you get older. It could even be that the initial infection is asymptomatic in many or most people and that only a small percentage of those infected ever get the later reaction. It could be one specific virus or it could be that several virus can do it. It could be dependent on some other factor such as how the virus integrates into the host genome or later immunosuppression.