The leading cause of reflux is the presence of a Hiatal hernia.
A hiatal hernia is the pushing up of the stomach into the chest cavity through a hole in the diaphragm. The presence of a hiatal hernia may hinder the action of the LES. Almost all persons who have Hiatal hernias will experience some degree of gastroesophageal reflux, thus it is the leading cause of reflux. If the esophageal hiatus is too wide, part of the stomach may bulge above the diaphragm. This bulge is called a hernia. If the LES is not tight, stomach acid may move up into the esophagus.
Read more about reflux.