Why is the trolls fighting in chapter 2 an example of slapstick comedy?

As the trolls fight over what they're going to do with Bilbo, a hidden Gandalph throws his own two cents into the mix and cause them further consternation. Each believes the other has said something to argue about, when it is, in fact, Gandalph. The fact they don't recognize an outside voice is comical, their actions would be classified as slap-stick. None-the-less, they argue over nonsense until the sun comes up and turns them to stone.