Their AI is weird. if you grab something like grass from them,they attack you. If you dig up their grass tufts, they wont attack you. If you light someones house on fire, they attack you. If you blow up someones house with gunpowder, they don't attack you, and so on.
The pigs also act weird to webber,not only guard pigs. the regular pigs are scared of him (but you can still trade with them) and the guard pigs attack him, even tho they have never seen a spider like him. Also webbers quotes towards the pigs suggest that they are better then the pigs in the constant because they dont attack him.
im not sure if them acting weird to webber is a bug or not, but anyway its better to post a bug rather then ignore it.