There are two common causes for steering wheel vibration, friction material deposits and disc thickness variation (DTV).
Some friction material technologies rely on an even layer of friction material transferred onto the disc surface. This is called a transfer layer. When this layer becomes irregular in thickness brake shudder will occur due to a grab and release effect from the pads on the disc rotor when braking. This oscillating torque from the brakes will transfer to the steering rack resulting in steering wheel vibration.
Disc thickness variation, DTV is an irregular wear issue with the disc rotor. As the name suggests, the thickness of the friction disc varies causing the pads to grab and release braking. Both symptoms result in the same effect but for different reasons.
DTV is typically caused by incorrect fitting of the brakes. The two most common fitting errors are not cleaning the hub mating faces correctly and irregular wheel nut torque, typically from the incorrect use of pneumatic impact wrenches.