P. 23 SO, and P. 80 BMM
The rules on dropping troops (or 'Mechs in BMM) say, that if a landing unit violates Stacking limits Unit Displacement happens per rules on p. 151 TW / p. 55 BMM. What is left unclear is which unit gets displaced. Is the landing unit displacing the one that was in the hex earlier (as it is technically the one entering the hex), or the other way around (the landing unit gets displaced, the one, that was already on the ground stays put)?
Also if a displacement happens as a result of a failed landing the rules say, to use the "original target hex" (the one, there lading was supposed to originally take place) to determine the direction of movement for any displacement. Does it mean, that the rules prefer displacement towards, or away from the hex, where the landing was originally supposed to happen? I would guess "away from the the original target hex" if the landing unit displaces the one already on the ground, or "towards the original target hex" if the landing unit is the one displaced, but am not 100% sure of that.