Dr. Packman About The Theory Of The Buteyko Breathing Method:

Someone who chronically hyperventilates is breathing too deep or too fast without realizing it. This causes a decrease in the body's carbon dioxide level which changes the body's pH balance. Then the lungs go into spasm as an attempt to hold on to carbon dioxide, in an ultimate effort to normalize the body's pH balance. This can lead to an asthma attack. 

Watch this video to learn more.