Typically, the failure starts with a small imperfection on a race. That generates excess heat, and destroys the grease inside the bearing or even causes some of it to escape.
This isn’t necessarily something the driver will notice right away, because it’s a bit of slow burn. Over time, as the ball bearings go over the race, the damage gets worse, and you can get conditions like brinelling or spalling.
This video is sponsored by the Automotive Distribution Network.