I'll attempt an answer: it's your flight-or-flight response kicking in, without any obvious reason for it to do so. Oxygen intake goes up, muscles tense up, adrenaline production kicks in, but there's nothing for your body to "spend" these physiological changes on (like running for your life), so it just sloshes around causing panic attack symptoms (eg sweating, trembling, shortness of breath, chest pain, nausea, detachment, fear of dying).