Possibly some kind of instinctual trigger. Think about the moment where the entire body was fully EXPECTING a lung full of air... and ready for it... but for w

Possibly some kind of instinctual trigger. Think about the moment where the entire body was fully EXPECTING a lung full of air... and ready for it... but for whatever reason you become submerged and cannot get it. You aren't unconscious yet because you still have plenty of oxygen, but your instincts have been triggered on a more primitive level -- possibly initiated as the lungs failed to function as expected.