This happens so often to me with the VC - everything seems to be going smoothly, I start telling teammates I'll get 15+ and last two turns the tide turns and everything is spiraling downwards. Currently I don't even try to push unless it's breakthrough. The dead are deceitful and can stack a good amount of buffs on long combats plus they have some of the best chaff units in the game.
Getting charged in the flank by zombies / skeletons can already seal the deal
Battle Reports in German (TTW): Spartans - what is your profession?