why is it dangerous to a swimmer to hyperventilate in order to hold breath for a longer period of time?

Hyperventilation causes CO2 levels to drop below normal, which causes the pH of the blood to rise. A rise in CO2 is what makes you want to breathe so hyperventilating lets you stay underwater longer. Eventually you reach a point where you have to breathe but there is no urge to because the CO2 level in your blood is depleted, you don't get oxygen rich blood into your brain and you pass out also known as shallow water blackout. Contrary to what another poster said, this isn't caused by too much O2, rather caused by a lack of CO2 in your blood. If you are going to hyperventilate at all I suggest only taking 2-3 deep quick breaths rather than doing it for a long period of time like others.

When you hyperventilate, you lower the CO2 levels in your body which normally gives you that urge to breathe after holding your breath for awhile. You can have a shallow water blackout when this occurs because you have little or no warning that you need air because that urge is not there. My breath hold record is 3 minutes and 27 seconds underwater but not by hyperventilating.

the danger exists when hyperventilating as you are blowing off excess carbon dioxide to resist the urge to breath underwater. this can lead to shallow water blackout, a condition that can occur underwater causing the swimmer to pass out from lack of oxygen. .

It's because of the risk of greying or blacking out.