Dont know if this really would take place in RL (this knock off the horse thing), that being said:
Saddles in Ed are not yet perfected (just like in RL history) to hold rider while he charge, but you need to hold spear/lance really hard to get maximum damage ( because if you dont you wont hit as hard), there is a chance to be knocked off the horse. But main reason to use lance (or long spear) on horse is their lenght.
In game terms: You will always strike first your opponent even if he has greater Initiative. The only thing faster (more Initiative) oponent can do is:
- get hit
- shoot and get hit
- hide behind something
- prepare for charge (if his spear is long enough, and most spears are much shorter than lance, or use pike - but its mostly useless in skirmish foot fights)
- and even if horseman misses you, he will turn slightly and ride by out of your reach (so good long spear is a must to fight lance using horseman, or you wont even have a chance to attack)
If you get hit by charging rider most of the time you are Knocked Down - which means you are no longer Prepared for the charge.
Riposting Longer Weapons p.165 rule is mostly unrealistic because if you have 6 meters long lance how on Passions the oponent can reach you with 2h sword (1.8 meter maksimum - more for troll weapons but still...).
But lances are one hit weapons because they shatter after successful hit (in RL), but can pierce even several men if they stand in tight formation.
So even if there is a chance of being knock off the mount with all that advantage IMB horsemen with spear (or even worse, lance) are feared. And use it. All the time.