We breathe faster at the top of a tall mountain due to a natural reflex. As we go higher up a mountain, the density of air and oxygen decreases, i.e. the amount of oxygen in the air decreases along with other gases. Therefore, to maintain the regular supply of oxygen, the body breathes faster. If we go too high without oxygen tanks, we could fall unconscious and die within 8 minutes due to oxygen deprivation.
P.S. This question should be in science section.