What behavior of the trolls caused their downfall in the hobbit?

After they caught Bilbo, the dwarves came looking for him. As they got around the fire the trolls popped a sack over their heads. It all started when they caught Bilbo. As they were questioning him, Balin came in, quite unsuspecting of anything. The trolls jumped him before he knew what's what. Then they hid behind the trees out of the light and waited for the other dwarves, who they promptly caught in a sack.