From one of the technical training manuals:
"Steering Angle Sensor
The steering wheel angle sensor is a potentiometer with two brushes positioned at 90 degrees to each other. The potentiometer provides data to the DSC control unit. When first fitted on the factory assembly line or when replaced in service, the sensor must be calibrated with the wheels in the straight ahead position. Thus, steering wheel angle is identified and analyzed by the DSC control unit."
Sounds like the DSC didn't know when the steering was pointing straight ahead, so it's information would have been conflicting with the pitch and yaw sensors that are mounted in a box behind the handbrake - that's how it knows there's a fault.
As for the other questions, I don't know what would have made go out of calibration (assuming it hasn't always been like this). Maybe it was just a bad sensor - they're only potentiometers, they could go bad. For the future... nothing you can really do but take the dealers warranty and hope you were just unluck with this component.

 

thanks