How does the fire go out in Part 2?

The fire goes out when the snow on a branch dropped onto the fire.

Now the tree under which he had done this carried a weight of snow on its branches. No wind had been blowing for weeks and each branch was heavy with snow. 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. This process continued, spreading through the whole tree. The snow fell without warning upon the man and the fire, and the fire was dead.