Why do I hyperventilate when I walk fast?

Hyperventilate. There are several possibilities. If you mean by hyperventilating you are short of breath with effort of walking fast then it indicates that you may not be getting enough oxygen to your heart. This could be a pulmonary problem or a cardiac problem. You must call your doctor to report this and be examined to find the cause and then properly treated. Do not postpone== this could be serious.
Processing oxygen. Your body is trying to process oxygen, as you're exercising. As your muscles repetitively contract, they require a smooth flow of oxygenated blood, to keep them working. As you get into better shape, the mechanism for processing oxygen becomes more efficient. When that happens, you may notice that you don't hyperventilate as much.