You know about the butterfly effect, right? And no, we're not talking about the mediocre (at best) Ashton Kutcher flick, The Butterfly Effect, from 2004. Obviously, most films don't really have the time or inclination to get into chaos theory and true quantum mechanics... but a small event cascading into a much larger event is a very popular method of big-screen storytelling.
Think about everything in Brazil only happening because of a typo. Think about everything in Pirates of the Caribbean: The Curse of the Black Pearl only happening because Elizabeth Swann's corset was too tight. Think of everything in John Wick only happening because Keanu Reeves's eponymous character didn't want to sell his car to a complete stranger at a gas station. Sometimes, the little things can lead to very, very big things.