What causes a car's brakes to "grab"?

Car brakes "grab" due to worn brake pads or brake discs, according to About Autos. A car can also grab if the brake pads are contaminated with oil or other fluids.

About Autos states that contaminated brakes may also occur if air gets in the system and forms condensation. Compromised brakes vibrate when gripping the brake rotor. Draining contaminated brake fluid and replacing it secures a clean brake system, but the car may also suffer from low brake fluid. If the brakes are worn, the car veers to one side when the driver stops suddenly. The brakes may also feel low if they are defective. Replacing the brakes is the best way to stop grabbing. Switching out the brake discs should be done in pairs only.