Double dosage of any chromosome in humans is typically not compatible with life or will cause severe mental and developmental dysfunction. The body and its development are fine tuned for the gene doses it receives.
In order for females to be healthy, the double X system evolved to self-deactivate the second X chromosome so a double dosage does not occur. The resulting inactive X chromosome is termed a barr body. You can also see this effect in female calico cats - the fur patterns are the result of some cells having one X active and other cells having the other one.
This paper states that the gene regulatory mechanisms that shut down the second X chromosome are the causal agent for this differential sex determined auto-immunity. Though the X deactivation mechanisms play a critical role, they can accidentally trigger the immune system to attack self.