And before the moment of hesitation had given way to action, a deed of Nature’s own violence overwhelmed them both. A fierce shriek of the storm had been answered by a splitting crash over their heads, and ere they could leap aside, a mass of falling beech tree had thundered down on them.
In this passage the tree falls on the two men. This happens right before the men are about to take action against each other. This acts as the complicating incident because the men are no longer able to shoot each other since they are stuck under a tree. They are still planning the other's death after the tree falls on them when they talk about how their men could make it look like the tree killed him.