Run a finger across a used rotor and you may feel grooves. Light grooving is normal wear; deep grooves - called scoring - mean something cut into the rotor. Here is what causes brake rotor scoring and what to do about it.
What scoring is
Scoring is deep circular grooves machined into the rotor friction surface by something harder than the pad material. Unlike the fine, even wear pattern of normal use, scoring is deep enough to feel and see clearly, and it reduces the contact area between pad and rotor.
The main cause: worn-out pads
The most common cause is running pads down to the metal backing plate - once the friction material is gone, the steel plate or the pad rivets grind directly into the rotor; see rotor thickness. That metal-on-metal contact carves deep grooves fast, which is why grinding brakes must not be ignored.
Other causes
Grit or debris trapped between pad and rotor, or a chunk of delaminated friction material, can also score the surface; see brake pad wear patterns. A stuck pad that no longer moves freely can dig in unevenly too.
What to do
Light scoring may clean up with resurfacing if the rotor stays above minimum thickness; deep scoring means replacement - see resurface vs replace. Always fit fresh pads with new or resurfaced rotors. Start with quality pads and hardware.