Some people with acid reflux have hiatal hernias and some do not. Hiatal hernias are not a pre-requisite for acid reflux, but a large amount of people with acid reflux DO have hiatal hernias. A hiatal hernia disrupts the location of the LES but due to the hiatal hernia the LES is pushed up and lies in the chest. This is a problem because the diaphragm is a large part of helping the LES to prevent reflux. Now the pressure of both the LES and diaphragm are not working as a strong unit. The hiatal hernia contributes to the reflux because of the decreased pressure.
Acid reflux can also be caused by a hiatal hernia due to the hernial sac. The location of the sac is near the esophagus. Acid gets trapped in the sac. Because the sac is so close to the esophagus, when the LES relaxes, it is easy to reflux.