At the fight outside the sparkmaster cave, my party approached from the south-eastern direction. We killed all the enemies and left combat. Moving a bit more towards the cave triggered two archers to attack.\
Even though I barely survived the fight with no deaths as it was, and adding in two obnovious archers would have been very troublesome... I have to admit that it's a bug and those archers should probably have been properly grouped with the rest of the encounter.