Got a few theories:
-Morale critical failure - This seems unlikely. They saw their Elector and the Knight of Dusk carving through the undead like they're made of paper.
-Unexpected resistance - This is possible, if the humans we overwhelmed on the gates turned out to be sleeper agents like the servants(this near Drakenhof they could probably afford to deadman their whole military), their corpses or the surrendered would have activated after some delay and the army and our Greatswords were suddenly assaulted by super strong pseudoghouls.
-Incompetence/Sabotage - Someone in the line of leadership got their directions wrong and so they scattered instead of marched.