The climax for me is somewhat misleading, because it isn't actually when Eckels comes face to face with the Tyrannosaurus Rex, which is what we anticipate will be the climax, but it is actually when Eckels comes back to the present and he realises how his crushing the beautiful butterfly when he went off the path has had massive repercussions in the future. We are reminded of Travis's words earlier on in the story:
"Step on a mouse and you crush the Pyramids, Step on a mouse and you leave your print, like a Grand Canyon, across Eternity. Queen Elizabeth might never be born, Washington might not cross the Delaware, there might never be a United States at all. So be careful. Stay on the Path. Never step off!"
By disobeying this cardinal rule, Eckels has changed the future, and it is as he and Travis and the others realise this that the climax of the story occurs and the falling action begins as we become aware of the fate of Eckels, combined again with "the sound of thunder" as he is shot by Travis.