Delayed charge results catching up to you. Several times I would pull off a perfect charge and nothing would die, then 5 seconds later (after most of my knights have been killed in the resulting melee) huge masses of men just collapse. Cavalry charges in M2TW have been massively boosted in their effect (but the bug keeps you from pulling them off; not the AI though). Cavalry have an 'insta-kill' against anything they charge except pikes, where it's the horses that get 'insta-killed'. There's even a debate going on about whether or not troops in skirmish formation stand a better chance of resisting cavalry than regular formations (less of them get killed in the charge but their combat ability in the melee suffers).

CA has already said they are interested in tweaking charges so I suspect they are not very sure themselves about how this uber battering ram of death is going to be received by us.