What happens to Eckels at the end of Ray Bradbury's story "A Sound of Thunder"? What is the cause of the final event?From "A Sound of Thunder"

Eckels is the hunter who panics and causes the permanent alteration of history in Ray Bradbury's science fiction short story "A Sound of Thunder." Going back in time to witness the hunt of the dreaded Tyrannosaurus Rex, Eckels strays from the designated path and, upon returning to the present time, realizes he has crushed a butterfly and it has stuck to his boots. The tiny butterfly's untimely death has caused a chain of events that includes visible changes in people, their language, and in the recent presidential election. After Eckels' discovery, the tour guide and hunter Travis adjusts his rifle and raises the weapon. Next is heard the sound of thunder, the original description of the sound of a charging T-Rex. But this sound of thunder is different: It is the sound of Travis's bullet killing Eckels, the fitting end for his unalterable mistake in the past.