As far as I see it, you walled your settlement off completely with those wall elements, locking the goblins out. And with no place to go, they will just keep spawning over the firepit and never move again. (Fun fact: even if you later go there with your army, they won’t move, so your soldiers can just stand next to the firepit, hit all those goblins up there and level all the way to… whatever the highest militaristic rank in East Laughford is 
)
It’s basically Goblin AI exploit/bug again