Probably just not enough airflow to keep the thing turning.
An airplane needs a minimum airspeed, so does a helo-rotor, a gyro-rotor, a windmill-rotor....
I assume every plant is designed to be most efficient over the typical year, so one would adjust the design for the average wind.
Some will cut out earlier, some later.
Best yet, when things go hurricaning, they stop them too
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