A common reason is that the front rotors have warped. Go check out WikiAnswers on the topic of "warped brake rotors".
Rotors should wear out smoothly in normal driving. Use of the brakes that generate lots of heat (hard braking, riding the brakes) will result in warped rotors.
Warped brake rotors........
worn front end parts (steering or suspension) could cause a car to shake when appling the brakes.
rear brake drums or rotors are out of round.
If the rotors are warped it can cause a shake when you hit the brakes.
yes
If it was big enough, yes
Vibration when applying brakes is most likely warped rotors. Another cause could be loose caliper bolts.
Possible broken a-arm, or bent wheel.
Tires,inner tie rod bushings, if the steering wheel shakes when brakes are applied - brake rotors warped.
if your riding your brakes,you may have warped rotors, otherwise its something wrong with your steering and/or alignment. hope this helps : )
have your front brakes checked
Your rotors need to be turned. They have become warped.