hiatal hernia : What causes GERD?

No one knows why people get GERD. A hiatal hernia may contribute. A hiatal hernia occurs when the upper part of the stomach is above the diaphragm, the muscle wall that separates the stomach from the chest. The diaphragm helps the LES keep acid from coming up into the esophagus. When a hiatal hernia is present, it is easier for the acid to come up. In this way, a hiatal hernia can cause reflux. A hiatal hernia can happen in people of any age; many otherwise healthy people over 50 have a small one.

Other factors that may contribute to GERD include

alcohol use
overweight
pregnancy
smoking
Also, certain foods can be associated with reflux events, including

citrus fruits
chocolate
drinks with caffeine
fatty and fried foods
garlic and onions
mint flavorings
spicy foods
tomato-based foods, like spaghetti sauce, chili, and pizza

http://digestive.niddk.nih.gov/ddiseases/pubs/gerd/