I had some time to dig through more RM books over the weekend and found the answer, it seems I wasn't careful enough in my study the first time around. (Although this might have to do with our translated German version of the rules also...)
- The Trample1 attacks done by riding/draft animals cannot generally be freely 'chosen' as a mode of attack by the rider.
- However, Trample attacks happen in some special cases 'automatically', i.e. the horse will use this attack once certain conditions apply. Namely, these are:
- For stationary attacks, if a Rearing2 attack ends up doing a non-minor critical damage, the animal will attack using Trample in the next round.
- For charging attacks, if a Ramming3 attack ends up doing a non-minor critical damage, the animal will immediately follow up with a Trample attack in the same round.
Thus, the conundrum of trample attacks is solved: while clearly being the objectively strongest attacks, they can't purposefully be 'triggered' by the rider and are more a matter of luck.
N.B. Horses also have a Biting4 attack, that can be chosen when the horse is attacking stationary. This biting attack is generally a bit weaker than the Rearing attack and doesn't give the chance of getting a strong Trample follow-up -- however, it allows the rider to attack as well, making for a neat little bit of tactical decision making on the side of the player.
1 Table used: Trampling
2 Table used: Crush/Fall
3 Table used: Ramming/Battering
4 Table used: Biting