what causes shaking when braking
More often, a steering wheel that shakes when braking is caused by warped rotors, which your mechanic may be able to diagnose with a simple test -- that is, by spinning the rotors by hand to feel for imperfections.
Having a car vibrate when the brakes are applied is a common problem for many car owners. Most of the time, this is a result of the front brakes being worn out, which is usually due to warping.