I have a right frozen shoulder. Any idea why?

Multiple Causes. There are many many causes for this but many times a specific cause isn't apparent. Learn more here: http://www.Theshouldercenter.Com/frozen-shoulder.Htm.
Unknown. We do not know the cause of a frozen shoulder. We know that it is a painful syndrome that limits shoulder motion. Most of the time it will resolve on it's own over five years. Injections and physical therapy will often help the problem resolve over 3-6 months.