The human beings have been disintegrated after a nuclear blast. This is made evident here:
The entire west face of the house was black, save for five places. Here the silhouette in paint of a man mowing a lawn. Here, as in a photograph, a woman bent to pick flowers. Still farther over, their images burned on wood in one titanic instant, a small boy, hands flung into the air; higher up, the image of a thrown ball, and opposite him a girl, hands raised to catch a ball which never came down.
The fact that it was a nuclear blast is made clear as the town had a "radioactive glow" and the dog that had sores all over its body.
What makes this story scary is the absence of humans. How the mechanical objects created by humans can very well continue to exist without people.
This story was particularly frightening in the year it was published (1950), because it was near the beginning of the Cold War when the possibility of nuclear annihilation was not only possible, but seemed likely considering the animosity the United States and Soviet Union felt toward each other.