This is a very common way of breathing. If you look at a person who’s breathing this way, you can see that their shoulders are kinda collapsed, they’re slouching, abdominal muscles and the back are completely relaxed. The chest is collapsed and inactive, and is pressing into the diaphragm, thus not letting it do its job properly. When the air comes in, the belly kinda literally “drops out”.

One of the main reasons for breathing this way is sedentary lifestyle and not enough physical activity. Another reason - deliberately wanting to make your chest look fuller.