It's not the absolute speed of the air, but the rate of change of the speed. A significant shear can put large bending moments on the rocket.
This.
The rocket has to turn its nose to the wind to keep going in the direction it wants, and large changes of wind direction/speed require large and fast attitude corrections, and well... the hardware has its limitations.
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I'd love to find a better source for the launch criteria for upper level winds and windshear. This is as good as I can find: https://www.nasa.gov/pdf/649911main_051612_falcon9_weather_criteria.pdf ... and it just says not to launch through upper level conditions containing wind shear... without any quantification. Frustrating really.
The criteria probably is the result of a simulated launch with those wind conditions: does the structure survive and are guidance targets met?