There are many different causes of spitting up in infants, which can range from mild to more serious. GERD, overfeeding, and milk allergy or intolerance are some of the most common causes. GERD is a condition that results in stomach contents flowing back up into the esophagus, often causing heartburn and projectile vomiting. vomiting. Overfeeding occurs when an infant takes in more calories than they need, leading to excessive regurgitation.