the long explanation is time consuming and technical (fluid shear, torque bias, etc), but in a nutshell: you're having no turning resistance for the front driveshaft, and creating infinite fluid shear. Fluid shear generates heat, causing the fluid to thicken and lock the clutch. Thats no problem if you intend to stay RWD forever, however, once there is a front driveshaft reattatched, and center differential action is required, you are now stuck with a locked clutch since the fluid and steel plates are so badly burnt and overheated.
quickie explanation:
http://ask.cars.com/2008/12/viscous-clutch.html
I'm sure DaveP will chime in or tell people to search, as this topic has been covered extensively.