tyrannicide said:
As far as I'm aware and as a general rule, mounted troops will dismount if they cannot move their steed for a certain amount of time. For instance when large amounts of cavalry are all huddled together.
It may happen at battle start if you're not quick enough (holding F1 to send groups off is priceless here), or if you're unlucky. This is one of the reasons I don't like large cavalry armies: they are rather tedious to command.
If I open the command interface, select mounted troops and click the "mount" button, I'll see such dismounted troops travel back to their horses. I'm not entirely sure if they finally manage to mount it, since every time I noticed it, I've had other things to worry about (the opposition).
It happens at the start of the battle.
It happens when you tell troops to hold position because they can't figure out that a Charlie Foxtrot is not a proper battle line.
It happens when one troop gets hung up on a rock and they all pile up behind him.
It happens when one troop gets hung up on a tree and they all pile up behind him.
It happens when you attack Steppe Bandits/Desert Bandits/or any mounted troop and they kill the front group but don't have brains enough to kill the horses in front of them to move or get at troops.
It happens when a Khergit dies and his horse stands there while troops start piling up into it.
It happens one troop gets stuck on a single are of a bank and all pile up behind him because the AI can't figure out that turing and going around an obstruction is intelligent.
It happens because you start out in the middle of a cliff and have to turn around, ride down, then go around, finding a bank to ride up, but it's too slow for some and they dismount. I'm sure Sun Tzu wrote that you should always advance your line onto steep cliffs and ravines because it helps with mobility.
It also happens when they can't push through your infantry line or their own line when trying to form your battle/charge line.