Could you accidentally click on Silvanus statue, when it happened?
I tested that, going back to the save before the attack. If I climb up to steal the idol I get attacked by the druids in the square, but it is not at all similar to the big attack.
That happened in the hollow, by the way, so somehow the scene moved from the square with the statue to the paths in the hollow, adding to the chaos, confusion and pandemonium.
So, ptobably a bug.
I also tried stealing a potion lying next to one of the worshipers, but that was just the usual "put that down" reachion.
