Once the leader fell, the rest of the Hemogoblin army was quick to follow. Especially after Cynthia slinked her way behind their lines and assassinated their mages, completely cutting off their near endless supply of troops.
The majority of the Hemogoblins were still following the previous tactic of throwing themselves at the enemy, without a single regard for their lives. However with their respawn suddenly cut off, this tactic quickly turned detrimental and just increased the rate at which the human resistance could cull their once impressive number.
It seemed, however, that some of the Hemogoblins were smarter than the rest, or perhaps simply more cowardly at their core. Once the troops closer to the back line realised their mages had been snuffed out, and seeing the headless corpse of the leader they once thought undefeatable, many of them simply decided to run for the hills.