Refer to figure.
Due to the dissymmetry of lift, the retreating blade must operate at a higher angle of attack to provide a balanced level of thrust across the disc. The retreating blade's angle of attack will be greatest when the blade is half way round on the retreating side. As forward speed increases further, the angle of attack of the retreating blade increases.

The retreating blade reaches maximum angle of attack and airflow separation occurs due to excessive flapping. Perhaps surprisingly, the stall begins not at the root but at the blade tip.

A good way to remember this is to think. More forward airspeed -> more dissymmetry of lift -> more flapping -> higher angle of attack -> retreating blade stall.

The speed of rotation of the tip of the rotor blade is high, generally in the region of 390-450 knots (658-675 feet/second), in normal operation of many helicopters. In forward flight the velocity of the effective airflow over the advancing blade, when it is at the midway position on the advancing side, will be increased by an amount equal to the helicopter's forward speed.

In general, the aerofoils used on helicopters are designed to be efficient at relatively low speeds, so compressibility effects will become apparent at relatively low forward aircraft speeds.