I'm no expert on this aircraft but I think it's because jet engines don't respond instantly to throttle changes because combustion is a continuous process, rather than a series of individual strokes in a piston engine.
If you just suddenly reduced the fuel input, you reduce the pressure in the combustion chamber and the flame might go out. Instead you have to do it gradually and wait for the turbine to reduce speed.
These days, the throttle just tells the engine controller what power setting the pilot would like to have, and the engine controller reduces the fuel flow at a rate that the engine can handle.
So (again, at a guess) 20 feet is how long it takes the engine to reduce power.