Indeed, there have been no rigorous attempts to define the basic science of UCTD. It is presumed that many of the same immunologic mechanisms that play a role in lupus and rheumatoid arthritis may be involved. Theories in those diseases include a genetic predisposition, which is subsequently triggered by some environmental factor, such as an infection, that is improperly handled by the immune system. This in turn causes the immune system to be "turned up high," or activated, when it shouldn't be and to target the tissues of one's body instead of foreign invaders

http://www.hss.edu/conditions_undifferentiated-connective-tissue-disease-overview.asp