A number of conditions are believed to cause GERD. Different conditions may be active in different individuals, or even the same individual at different times. Chief among these are:

Lower Esophageal Sphincter (LES)
The esophagus is a muscular tube extending from the lower part of the throat to the stomach. At its lower-most end, where it joins the stomach, it has a muscular ring called the LES. It is the proper functioning of the LES that is most important in preventing reflux. The LES muscle is active almost continuously. In order to prevent reflux, it contracts and thus blocks off the passage from the esophagus to the stomach. After a swallow (of food or saliva), it relaxes for several seconds, thus allowing the food to pass from the esophagus to the stomach, and then contracts again.

In patients with GERD, two abnormalities of the action of the LES can occur. The first is when its contraction is weaker than normal, thus allowing reflux to happen more easily. The second is when its relaxations last longer than normal. These are called transient relaxations. These also cause reflux to occur more easily, especially after meals, when the stomach is distended with food.

Hiatal Hernia
In a normal person, the LES is located at the same level where the esophagus passes from the chest through the diaphragm and into the abdomen. In case of hiatal hernia, a small part of the stomach is pushed up through the diaphragm. As a result, the LES is no longer at the same level as the diaphragm a small part of the LES and stomach lie in the chest.

Normally, the combined pressure that is exerted by the diaphragm and the LES help to prevent reflux. With a hiatal hernia, both the LES and diaphragm continue to exert pressure and thus block off reflux, but they do so at different locations. Hence, the pressure exerted is weaker because it no longer has the combined strength of both. This causes reflux to occur more easily.

Esophageal contractions
Swallowing contributes significantly to the removal of acid from the esophagus. When we swallow, waves of contractions begin to travel down the esophagus, pushing along with it the food and saliva from the esophagus to the stomach.

A defect in the wave of contractions renders ineffective the pushing back of acid into the stomach. Sometimes, the wave may not begin after each swallow or may fade out before reaching the stomach. The pressure generated by the contractions may be too weak to be effective.

Prolonged Emptying of the stomach
The stomachs of some patients with GERD empty slower than normal after a meal. This prolongs the distention of the stomach after a meal. Hence, there is more time for reflux to occur.