Why does the DC-9's antiskid system have to be manually disarmed during landing rollout following a ground-shift-mechanism malfunction on takeoff?
Historical yet pertinent note: Concorde was the first plane to have electronic anti-skid, which means the DC-9 (also electronic) had the system installed after entry into service.



The relevant patent for this answer is: Anti-skid control system for aircraft.

The nose gear ground shift is part of the anti-skid system (see below; click to view).


  
  
  Source: DC-9 FCOM


Here's how the circuitry works:


In flight (mode), the ground shift fakes a fast wheel spin, but with no actual wheel spin, the circuitry then thinks there is a lock, and sends a brake release signal. This ensures the plane doesn't touch down with the brakes on.
After touch down and wheel spin, speed transducers send AC signals, which are converted to DC for the circuits.
Once the wheels are above 10 MPH, the brake release signal is removed, i.e. braking is now allowed.
Once firmly down with the gear compressed (nose gear in the DC-9), the squat switch switches off the fake signal. However, that takes full effect only when the circuit is de-energized, which happens when the wheels slow down below 23 MPH (20 knots).


In the case here – with the plane still in flight mode due to the pulling of circuit breakers after the malfunction – any time the plane is stopped (or circuitry is de-energized), the circuitry would release the brakes (thinking there's a lock).



Note: The newer MD-80 has the same procedure minus the manual disarming of the anti-skid, which hints at newer circuitry.