Belly fat enters your body in the same manner that fat from any other location does: through an excess of food. You should anticipate to develop some belly fat throughout your bulk because the essence of bulking is to eat calories at a caloric excess. However, keeping up with your training and monitoring your nutrition will help you reduce the amount of belly fat you develop. The more muscle you have, the less likely it is that you'll gain weight primarily as belly fat. The reason for this is that people who are muscular burn more calories per minute than people who aren't muscular. In addition, the more muscle you have, the lower your resting metabolic rate is, which means you need to eat fewer calories overall. This is why people who bulk often see their bellies shrink as they lose weight! There are several factors that may lead to increased belly fat during a bulk. First, if you're eating a lot of carbs and not much protein, your body will turn them into glucose which it stores under your skin in the form of glycogen. These sugars are then used for energy so they can't be absorbed by your digestive system and thus reach your bloodstream. Thus storing them under your skin allows your body to use these nutrients instead. However, if you don't exercise or don't keep up with your training, this sugar storage will occur even if you're following a low-carb diet.