There is an explanation that just came to my mind: everyone says that the Goblins will disperse if their leaders fall. Maybe the loss of their thinking heads make the goblins regress to a more feral state (I'm still not convinced though because once the war drums are destroyed how do gobbos out the inner sactum knew of the dead bosses? Even when one of the kids was able to flee from bear-halsin they weren't allarmed).
