Denial is typical of humans, and the author uses it to make the story more effective. In a crowd the stoning can be justified by each person present because they can tell themselves that they didn't kill Mrs. Hutchinson. They only threw one or two rocks. Everyone else killed her. This kind of phenomenon accounts for deaths in British soccer matches every year.
People fall and are trampled to death by other human beings.
Since we are familiar with this side of human nature, its appearance adds to the effectiveness of the story. The story is made more effective because the victim in the story is one of the more developed characters. Mrs. Hutchinson and her husband are two of the people we meet early in the story. We identify with them because most of the other 300 townsfolk are faceless to us. The story is too short to develop too many characters, so we identify with those characters that are more developed.