It never triggers if you start the combat from the cave mouth and fight with the Zhent. It *usually* (but not always) triggers if you attack the pack from the rocky outcrop or the mountain pass. It should happen mid-combat, basically as soon as Flind sees you.
I love the way fights start differently depending on which direction you approach them from. See also Gimblebock and co outside the crypt - four directions, four different outcomes!
