The rotor blade is slower at the root than at the tip. As the helicopter’s speed increases, the forward speed may exceed the rotor speed near the root. In this portion of the disc, the airflow will be reversed.
Although the blade is not stalled, it is not generating lift. As a result, the other sections of the blade need to provide more lift to equalize the lift on the rotor disc. The angle of attack (AOA) must be increased on the retreating blade, but there is only so much increase in AOA before a blade will stall.
Principles of Helicopter Flight, 2nd Edition, pg. 98