Leaves have formed the circumference of the circle

It is quite common for air to have approximately circular currents. Typhoons and hurricanes, and cyclones in general, are the same thing on a much larger scale.

 

Look up "turbulence".

When air flows past a corner, for example, some of the flow will curl around creating eddies. You can also get different angled breezes adding up to some circular motion and groups of objects can funnel a breeze into a vortex. As long as there is a breeze, these motions can be sustained for quite a time.

 

There are two effects balancing each other out. There is centrifugal effect forcing the leaves apart, and there is a ground air current pulling the leaves towards the center. At a certain radius, these two effects balance out. The reason for the ground current towards the center is explained by the Tea Leaf Paradox. Note that in a fluid, leaves are near-buoyant, so centrifugal effect is relatively insignificant.