Round table

An interesting situation has arisen. I was fighting regular harpies. Two groups of harpies flew up to a ballista and stood near it, as they usually do with archers. As if blocking it. But the ballista didn't care! And the harpies, apparently, weren't really aware of this. And, what's characteristic, they weren't even attacking it. They were just defending. The question is: is this always the case, or did I just encounter some exceptionally unintelligent harpies?