Actually this seem to be a more general problem as I have also seen this behavior in the fighter companion (using the aggressive rule set) and in enemy A.I., in the case of the enemies it seems to occur whenever enemies of different categories exist in the same map, for example forest trolls and human bandits really seem to hate each other as I have seen an a forest troll disengage and wander off to a bandit camp where I later found his corpse and then when I was fighting the bandits in question one of them wandered off and started fighting an another enemy in an undiscovered part of the map.