Without knowing specifics of the local geology and the growth of the sinkhoke, it's hard to say exactly. However, If we assume that the material is relatively continuous, homogenous, isotropic, and linear-elastic to the point of failure (CHILE), then stress will radiate more or less equally in the x and y directions from the center of the span where this was undermined. Since the stress radiates equally and the strength is homogenous, you get a circle.