The cause of Kussmaul breathing is respiratory compensation for a metabolic acidosis, most commonly occurring in diabetics in diabetic ketoacidosis. Blood gases on a patient with Kussmaul breathing will show a low pCO2 in conjunction with low bicarbonate because of a forced increased respiration (blowing off the carbon dioxide). The patient feels an urge to breathe deeply, an "air hunger", and it appears almost involuntary.
A metabolic acidosis soon produces hyperventilation, but at first it will tend to be rapid and relatively shallow. Kussmaul breathing develops as the acidosis grows more severe. Indeed, Kussmaul originally identified this type of breathing as a sign of coma and imminent death in diabetic patients.