What causes his second fire to go out ?

In Jack London’s “To Build a Fire,” the man builds three fires. The second fire goes out because the man makes a mistake: he builds the fire under a pine tree. Although this makes it easier for him to collect sticks to feed the flames, it ultimately proves fatal. Snow has been collecting on the top of the tree’s branches, and every time the man has pulled a stick, the snow has slowly worked its way down the tree, falling from one branch to the next:

Each time he pulled a stick he shook the tree slightly. There had been just enough movement to cause the awful thing to happen. High up in the tree one branch dropped its load of snow. This fell on the branches beneath.

Eventually, this snow falls onto the fire itself, extinguishing it and leaving in its place “a pile of fresh snow.”
This is how the second fire goes out. He does manage to a light a third fire, which is also referred to as “the second time.” This provides a sense of repetition in the man’s struggle against the harsh environment. At this point, however, the man is hardly in control of his own body, and he ends up badly burning himself. He is unable to start a fire with the small pieces of burning grass. Both of these failures depict a man that does not truly understand nature. The man is aware of facts, but he lacks the form of knowledge that the dog has—an innate knowledge of how to survive. Because of this, he mistakenly builds the second fire under the tree, and he pays the ultimate cost.