Transponder suppression.
The transponder is suppressed on ground.
Most types are switched on/ off on nosewheel squat switch.
If you dont believe me watch the guy racing down the rwy on takeoff next time you are on short final!
You can see him but TCAS cant. And voila he lifts, and there he is.
Same on landing behind a fella: Now he is there, now he is gone, ie on the runway, no more mode A and C.