What causes the conflict at this point in the book is what causes many of the conflicts between Ralph and Jack. They have different views of what their little society needs in order to survive.
Ralph is worried about the signal fire and the fact that it is near where the beast supposedly lives. He really thinks that they need the fire so as to be rescued. Jack offers his hunters as guards but Ralph does not respect them. He thinks that they are just irresponsible kids who are playing around and not serious about the important things.
This leads Jack to get angry. He thinks that his hunters are the most important part of the society and he thinks that Ralph should give them more respect and do more of what Jack wants him to do.