The muscle at the end of the esophagus is called the lower esophageal sphincter (LES). The LES is a one-way valve that normally opens for limited amounts of time when you swallow. Acid reflux occurs when the LES doesn’t close properly or tightly enough. A faulty or weakened LES allows digestive juices and stomach contents to rise back up into the esophagus.
Large meals that cause the stomach to stretch a lot can temporarily loosen the LES. Other factors associated with reflux include: