Imagine a stream of water in a channel flowing uniformly. This is undistorted flow. Now imagine the stream stops and starts. Now imagine also a block placed at the edge of the stream. Now the flow behind the block contains eddies, and swirls and is not constant with time.
The stopping and starting is wind gust variation. The block at the side creates an effect like the edge of the inlet, when the wind approaches from the side. The front of the engine sees eddies, swirls (turbulence) and non-constant (unsteady) flow.
Engines are tested to demonstrate resistance to this. And they are often tested with a screen to eliminate this distortion, when it is specifically important to have uniform inlet conditions. See here at 1:20
The most common type of inlet vortex is formed when the inlet is near enough to the ground that the inlet flow coupled with a crosswind shear flow forms a vortex. There must be shearing (cross flow) to initiate a vortex. Just like a tornado is formed by shear flow. See here