Why is it not recommended to hold the nose up on the A320 after touchdown?
In flight mode the stick commands a load-factor. Which means it will be impossible to flare the aircraft, because as you pull on the stick, you'll be commanding a positive g-load.

Because of this the Airbus has a flare mode which activates at 50' RA. At 50' the pitch angle is stored (memorized). At 30' the aircraft commands a 2° nose down (it takes 8 seconds). The pilot would gently counter this by pulling back, resulting in a nice flare and no increase in the auto-thrust.

If the pilot did not float for way too long (they should not), the plane will touchdown before the 8 seconds pass, further eliminating the need to push forward on the stick to keep the nose gear planted.

The logic reverts to ground mode when two conditions are met: on ground for 5 seconds and pitch angle &lt;2.5°.

If the 5 seconds pass and the nose is still +2.5°, the logic will remain in the flare mode and the THS will not reset to 0°. The pitch will revert to the one set at 50'. As the plane continues to slow down this may result in a nasty nose landing gear slam or a tail strike depending on the stored pitch angle and any gusts (kind of like this funny TV ad).



I do recommend you check your company's Flight Crew Operating Manual (FCOM) and to not use the internet for flight. If your instructor does not like too many 'why' questions during class, you can try and ask them after class. When you get your answers, try to validate them from the FCOM.

Some companies that fly extreme approaches have custom logic set by Airbus. So the installed logic may differ.