The AOA of the retreating blade varies across the span of the blade. The inmost foot or two is stalled most of the time. This stalled region spreads outward from the root toward the tip as the aircraft's airspeed increases. A sudden, full stall of the entire blade in steady flight is unknown.
A full-blade stall can occur if (1) sufficient RRPM is lost during a low-G maneuver, followed by a sharp increase in disk AOA; or (2) if a violent nose-down rotation of the airframe, with spindle fixed, occurs, suddenly increasing the retreating blade's AOA.
What gyro pilots call "ground flapping" is a special case of Scenario #1 above, except that the low RRPM is not a result of a low-G maneuver, but instead results from insufficient pre-rotation during the takeoff sequence.