Jellycopter,
My speculation from experience of other designs; if the T/R control became disconnected, the blades would revert to a pre determined position by design. This position is a balance between aerodynamic and centrifugal turning moments which are usually designed to apply some positive pitch to the blades to allow a running landing. In a vertical climb OGE, this pitch setting would not be sufficient to prevent yaw developing (quite rapidly).
That might be true for smaller helicopters but if there is a flying control hydraulic system driving the tail rotor servo to either full positive or full negative pitch, that can't possibly happen. Some aircraft do have a centreing device to hold the servo at a pre-set mid range pitch position if the flying control run disconnects (e.g. control cable break), others don't. In a situation where the control mechanism fails, any number of things might happen. If the servo puts the tail rotor pitch to a medium or neutral position, you might be able to control the aircraft in yaw by either varying the main rotor torque, by changing the airspeed, or a combination of both. If the tail rotor pitch ends up at either full positive (anti-torque) pitch, or as possibly happened in this case, somewhere near maximum negative (pro-torque) position, you are in a very dire situation indeed. The fixed wing equivalent would be a rudder hard over and an engine failure on the same side that the rudder surface had moved to.
To put some typical figures on this, an aircraft you might remember very well (Puma) has a tail rotor pitch range of something like +35 degrees to -17 degrees astride the "zero pitch" datum. The RAF HC1 had no tail rotor pitch centreing device fitted, whereas the civilian Super Pumas did.