when rotors are warped means that the disc brakes are not straight anymore, and when they are spinning a section of the rotor gets closer to the pad then the rest of the disc, this causes the steering wheel to shake,
so basicly when the disc is moving through the brake caliper a section of it isnt even and sort of slows down faster then back to normal, then faster again
hope u under stand that

warped rotors are cause by u over heating the disc, and not letting them cool properly, sometime can be caused if u heat up the brakes alot then just park the car, the rotor cools down but the bit in the caliper doesnt, ie gets warped