Question

When strong wind blows, and umbrella held upright at times get upturned. Why?

Answer 1

When we hold umbrella upright in our hand  the wind blow from high pressure area to low pressure area so from under the dome of umbrella high  pressure wind which blow upwards to low pressure area by touching edges of umbrella hence it gets upturned.    

Answer 2

an umbrella has a small surface ad a not-so-perfect aerodynamic shape. In order to generate enough lift to send an average person flying, it needs winds so strong that will tear it apart.