How does the fire become uncontrollable in lord of the flies?

Because the boys were very enthusiastic and decided to put much more wood to the fire signal.

Explanation:

In the story, the boys were so excited because they managed to make a fire signal that they decided to make it a little bigger, they were all so excited and started putting up more firewood until the signal turned into a bonfire.

As the wood burned, large sparks came out, which in dry weather soon started other fires around the surrounding jungle until all the fire got out of control quickly creating an uncontrollable fire.

The fuel that they used to begin the fire was not good anymore, or like expired, so it burns like crazy. the boys feed it more and more wood to keep it lit but it then it dies out fast because of the rotten fuel.