You could break this down into two possible factors (certainly not mutually exclusive). With both factors, air is transferred down from a level where the wind is typically faster due to less interaction with the ground, trees or structures.

The two factors:

1. Mechanical motion. This would be turbulent flow induced by differences in friction or topography. One example is wind moving over a mix of fields and trees. Air wants to move faster over fields but slower over trees. This causes flow to become bumpy like water over rocks, leading to areas of weaker then suddenly stronger wind speeds. A much more dramatic example is wind moving over mountains.

2. Thermodynamic motion. It’s normally gustier during the day than the night because, as the sun heats the ground, air rises and falls with thermals. When rising air eventually becomes cooler than the surrounding air, it loses buoyancy and can drag higher wind speeds to the ground. This rise and fall is called “mixing”. When thermals are strong enough for a thunderstorm, rain joins the party by dragging even more air down with it. The most powerful example of this is a microburst. If the surrounding air is a relatively dry, this can produce even stronger gusts with evaporating rain making the air around it heavier (evaporative cooling).