Having spotted that Bilbo was in trouble, Fili and Kili had made the decision that they would need to go back and get the others anyway. They couldn't leave their burglar to be eaten by trolls, that wouldn't be fair.

"Drop him!" Kili cried as he leapt out of the bushes, his sword held aloft.
"You wot?" exclaimed one of the trolls, looking rather bewildered by the sudden appearance of the young dwarf.
"I said, [i]drop  him[/i]!" Kili repeated forcefully, twirling his sword around as he glared up at the trolls. With their own shouts and cries, the other dwarves also leapt from their cover and charged at the trolls.