In chapter 2, the boys run off to get wood for the fire.
After the boys gather to discuss their situation, it becomes clear that there are no adults around to help them. In order to get rescued, the boys need to make a signal. They decide they need to make a fire as a smoke signal.
Jack goes off to find wood for the fire, and the other boys follow him before Ralph can stop them.
The space under the palm trees was full of noise and movement. Ralph was on his feet too, shouting for quiet, but no one heard him. All at once the crowd swayed toward the island and was gone–following Jack. (ch 2)
Basically, the boys want to do something, and they find looking for wood interesting. As long as something is novel and fun, they will do it. Yet there is not really order and focus. Ralph is not able to act as a leader, and Jack is only undermining Ralph.