As mentioned in my previous posts is where the answer lies. All you are doing with the hard breaking is stressing the moving components, especially the guide pins if there are dirty or slightly corroded, you are using the antilock braking control to it's maximum possibly subject to how hard you are breaking.
Unless you decide to strip it down and look you will not find the cause.
Also I know this because the same was on my mini the left rear, I had uneven wear and grooves across the disc, all other wheels were perfect. I did the hard braking approach which cleared it up enough to satisfy myself until I come to a pad change, my brakes have done less than 9k miles. When I come to change my pads and discs I will monitor the components as I strip it down to change them.
Always said I would go back to a 6 series