What is the cause of the final event in the story "A Sound of Thunder"?
"A Sound of Thunder," a short story by science fiction master Ray Bradbury, describes a group of hunters who go back in time to kill a Tyrannosaurus Rex. However, the trip goes awry when the hunters stray from the assigned trail and kill several objects that were not meant to die at that time. It has already been explained to them that even the slightest change in the past can cause a domino effect of change in the future. When the men return, they notice drastic changes around them, particularly the recent presidential election in which the original loser has now triumphed.
As one of the men drops to his knees, pleading to return to the past in hopes of fixing the future, he hears the other man "shift his rifle, click the safety switch, and raise the weapon. There was a sound of thunder."
The sound of thunder, the phrase originally used to describe the sound of the dreaded T-Rex, has now become a rifle shot that ends the life of the disgraced hunter.