Level 1: Reasons why babies keep spitting up milk. It is a very common phenomenon that babies keep spitting up milk. There may be many reasons. First, some babies may eat too quickly or overeat, which can cause stomach upset and cause vomiting. Secondly, if the baby\’s posture is incorrect when feeding, it can easily cause vomiting. In addition, some babies may suffer from digestive diseases or allergies, which may also cause vomiting.