If the latter represents typical fairy circle formation, then circles must grow over several years, because their initial size would be much smaller than is typical for the area. (journals.plos.org)
Human soil disturbance such as fences and roads can change circle shape, and possibly initiate circle formation. (journals.plos.org)
As fences were probably also associated with vehicle tracks, perhaps some kinds of soil disturbance stimulate circle formation. (journals.plos.org)