It is not Crohn's related, it is auto-immune disorder related. Both Crohn's and rheumatoid arthritis are manifestations of autoimmune disorders and patients commonly have both and/or other autoimmune disorders. In fact there is a thread about multiple autoimmune disorders going on right now. The good news is that biological drugs can be used to treat both. Humira that works for Crohn's often works well with the RA. I have a friend who has both and he says his Humira treats both.
See this thread: