Aerophagia -- excessive swallowing of air -- produces belching or burping. The upper gastrointestinal gas that erupts from the mouth comes from swallowed air that forces itself back up. There are actually two kinds of belching. One is a fairly innocuous condition that stops on its own about an hour after eating and is caused by air swallowed while eating. The other is more persistent and is caused by constant air swallowing, a nervous habit most people don't even know they have.
Technically called eructation, belching occurs when the upper esophageal sphincter relaxes and lets the pressurized gas that has been swallowed escape from the mouth. A study has found that healthy young people belch an average of 11 times in 20 hours, excluding mealtimes.