That is you removing that layer of rust that occurs on the rotors when parked overnight.
If you modulate the pedal just right, you can make the most horrific sounds. But remember this is only while in reverse.
Here is an explanation I wrote in a previous post:
Yes, the problem is from oxidized material on the rotor surfaces. It is normal, and is common with newer vehicles which use metallic brake pads, and rotors.
High humidity speeds up the process too.
I have noticed it on several of my cars, and can say that it will be louder when backing up then when going forward. Therefore my theory is two parts.
1. Since we apply the brakes while going forward 99% of the time while driving the vehicle, the brake pad surface will be fatter at the trailing edge of the pad and thinner at the leading edge of the brake pad. This process is normal, and is just a physical effect from the direction of motion of the rotor, and the leverege of the brake pad in the caliper assembly when brakes are used.
2. The buildup of oxidation on the rotors, will be scraped off without much noise when going forward, but when going backwards, (i.e. backing out of a garage) this uneven brake pad surface is now going to be wedging the opposite direction in the caliper assembly creating a poor contact between the brake rotor and the brake pad surface.
This larger than normal clearance in the rotor to brake pad contact will trap the rotor rust under the brake pad surface, (instead of being scrubbed off if you were going forward). The trapped rust will vibrate against the rotor, and cause the squeal/groan. The problem is further enhanced because when we are backing up, we tend to apply the brake pedal softer.
Does this make any sense? Hard to explain without a diagram, props, and my little pointing stick!