Because brake rotors are clamped by the caliper and brake pads during deceleration, it is not uncommon for brake rotors to become scored over time.
Dirt and Debris
One cause of scoring on a brake rotor (also called a brake disc) is dirt and contaminants that have become trapped between the brake pad and the surface of the rotor, causing a groove over time.
Worn Brake Pads
Another cause of scoring on brake rotors is pads that are beyond their serviceable life. If the brake pad material wears away completely, then metal is exposed that begins to wear on the rotor, eventually causing scores.
To prevent your rotors from scoring, inspect the brake pads regularly and replace them if they are worn. Also, regularly cleaning your rotors with brake cleaner may help prevent the build-up of dirt and debris.
Determining Rotor Scoring
To determine if a rotor is scored, remove the wheel and run your finger over the rotor's surface. You will be able to feel whether the rotor is scored. Make sure that the brakes are cool before you do this.
If you have a badly scored rotor, you can remove it and take it to a machine shop to have it resurfaced for about £19 if the rotor is still thick enough to use. If the rotor is beyond the manufacturer's recommended tolerance, then you will have to replace it.