Natural spiral pattern of grass - what could cause this?
I came across this photo from Mount Calavera, San Diego County, California.  It is described as rock art and is also found in the floor of a quarry.  The spiral pattern, including the vegetation, is very similar to your photo.  In your photo it looks like the rocks have just been removed.



Rocks appear to affect the soil moisture available for vegetation by shading. From a study in Southwest China investigating the influence of bare rocks on soil moisture:

•We found that bare rocks influenced the surrounding soil moisture by shading.

•The effect degree of rocks to surrounding soil moisture correlated with its shape.

•Soil moisture on the north side of the rocks was significantly higher than others.

•The location at 15 cm north of the rocks was ideal site for plantation restoration.

Source: http://www.sciencedirect.com/science/article/pii/S0341816213003123

Is it possible that rocks used to be placed in a spiral pattern in your photo that allowed the vegetation to take hold?