Wind shear is a sharp change of wind speed or direction over a short distance. It most often forms at a boundary between two air layers — for example between a calm ground layer and fast-moving air just above it.
A drone climbing out of the calm ground layer into the fast air above can be flipped or shoved sideways right at the shear line. Shear is strongest near thunderstorms and around buildings, where the transitions between airflows are sharpest.