Because you have a vacuum leak somewhere in the power brake booster. If you take and plug the hose that goes to your brake booster from the intake manifold the problem will go away. Be carefull, because your braking ability will also be severely compromised. This means that you may have a hard time stoping the vehicle. It is likely that you havea crack in the diaphragm in your booster. I am guesing that you also have a change in idle when you appl the brakes. If this is indeed the problem, you have to change the diaphragm or the whole booster itself.