People burp when drinking soda because soda is basically carbonated water. Going into chemistry, soda has a chemical formula of H2CO3, which is carbonic acid (aka. Hydrogen carbonate). When pop isn't fizzing, it usually is enclosed with a large amount of pressure; opening it starts a chemical reaction, changing H2CO3 to water and carbon dioxide. The body naturally wants to take carbon dioxide out for 2 reasons: it's a gas and it's what we generally exhale anyway when breathing. Burping occurs when that carbon dioxide finally escapes the body.