Why does the DC-9's antiskid system have to be manually disarmed during landing rollout following a ground-shift-mechanism malfunction on takeoff? An aircraft's ground-shift mechanism (usually a sensor wired to the nosegear strut that looks to see whether the strut is compressed or not) tells the aircraft whether it's on the ground or in the air.  This determines how the aircraft responds to certain situations; for instance:


Many aircraft's spoilers1 can only extend to the full-up ground-spoiler position if the aircraft is in ground mode.
Applying TOGA thrust with the flaps up triggers the takeoff-warning horn in ground mode but not in air mode, whereas the stall warning, in contrast, can only sound in air mode.
Reverse thrust, on most aircraft so equipped, is generally locked out in air mode, and the minimum thrust available changes from the ground-idle setting to the considerably-higher flight-idle setting when the aircraft lifts off.
The outflow valve on most pressurised aircraft automatically goes to the full-open position when the nosegear strut compresses (this depressurises the cabin, allowing the doors to be opened when the aircraft reaches the gate).


The ground-shift mechanism can occasionally malfunction, in which case fun things can happen, like the engines refusing to reverse upon landing and the stickshakers going off during taxi (if it fails in the "air" position) or the takeoff-warning horn going off when you raise the flaps during climbout, the landing gear staying locked down, and the aircraft refusing to pressurise (if it fails in the "ground" position); the usual workaround is to force the aircraft into one or the other mode using circuit-breaker fu.

One specific type of aircraft (the DC-9) has the following interesting entry in the abnormal-procedures section of the AOM, regarding what to do during landing if the ground-shift mechanism failed upon takeoff:


  Approach and landing:
  
  If landing gear was not retracted prior to landing, ground spoilers must be operated manually.
  
  AIRPLANE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. REPRESSURIZE (PNF)
  
  
  Ensure airplane is repressurized prior to landing.
  
  
  ANTI-SKID SWITCH (before 30kits ) . . . . . . . . . . . . . OFF (PNF)
  
  
  During landing rollout and prior to 30 kts, momentarily release brakes and place Anti-skid switch to OFF.
  
  
  GROUND CONTROL RELAY C/Bs (if pulled) (H20 and J20) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . RESET (C or FO)
  
  
  Reset Ground Control Relay circuit breakers
  during taxi and verify that circuits are in the
  ground mode.
  
  
  [NTSB AAR-96/07, page 134 (page 144 of the PDF file of the report); my emphasis.]


Why does a ground-shift-mechanism failure require the DC-9's antiskid system to be manually deactivated during rollout?



1: (Though not all.)
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.