Why do my rear breaks make a grinding high rubbing sound? on 2008 Mazda Mazda5

When in reverse only the rear breaks make a hard grinding rubbing noise. The dealers says breaks ok. But still not acceptable. Mileage 85000 km. 4years same brakes

Asked by for the 2008 Mazda Mazda5
How many miles (km) since it was last checked? Hard for us to argue with a dealer that can look at the vehicle!
78000 also rubbing noise rubber on rubber almost like the brakes are sticking on