Square Leg is almost there I think!
If ADIRS 2 (and or 3) is u/s then with a subsequent ADIRS 1 malfunction the SFCC can 'see' an overspeed and do an auto-retract at any time. This actually happened in the early days of the a/c I believe and resulted in the flaps going to zero just after rotate!
By selecting Conf2 the auto-retract is effectively diabled just as SL has pointed out.