Breathing is vital in sports. It brings oxygen to the body, fuel providing energy to muscles. When the level of oxygen is sufficient to make muscles work, its called aerobic zone.
When the effort becomes more intense, the athlete reaches a point where oxygen is lacking and enters into the aero-anaerobic zone. The body has to cope with a shortage of energy, producing it through other methods and creating lactic acid, which has to be destroyed. This process creates more CO2, and the breathing has to intensify significantly to get rid of it.