Basically, they underestimated the number of Tuskers that survived the first fight; the company is largely intact, nominal casualties. They expect a few desperate orcish holdouts, depleted, not an intact whole. 

This massive underestimation wont be a thing after this battle, basically. They hired orcish mercenaries. They never really got that they were Orcish mercs looking to figure out how to fight them, ruthlessly exploit the weaknesses, and use every advantage handed to them. It works especially well against arrogant bunnies that think that Nar Mat Kordh-Ishi should fight a certain way because that is how orcs have always fought.