I think some are probably more prone to freak from this than others. I run into the water-up-the-nose panic during scuba classes and we teach mask removal skills. It's one of the reasons I always emphasize to the students to flood the mask from the top rather than the bottom. Even then, even with control and being able to breath, about one in every class will flip out and stand up the second they get a little water up their nose. I can easily see how panic can build in an inverted position like this where the dead air is staying in the lungs because of that position (it's CO2 buildup that gives your body the breathing reflex, rather than lack of oxygen) - even if you breath out to prevent water from entering the nose, you're breathing out the good air, doing nothing to satisfy that breathing reflex, and on top of that you've got the drowning reflexes...The more nervous you get, the faster your body uses oxygen....Yeah...game over.