According to WebMD, the medical term for burping is "eructation." The site reports that the air in eructation originates from either the stomach or the esophagus, and the action is sometimes accidental and sometimes purposeful.
WebMD lists several of the potential causes of eructation, including the swallowing of air during a meal, a nervous habit or medical problems, including ulcers and gallbladder issues. In some cultures, eructation is recognized as good etiquette after having been served a meal.
The National Center for Biotechnology Information suggests that gastroesophageal reflux disease --GERD-- also leads to eructation. Patients with this disease often suffer from acid reflux, and eructation is considered a possible symptom.