Sure looks like chemical burn.
Funny, in less than a weeks time, the plants were devoid of chlorophyll, like a systemic plant killer was used. The use of weed killer would explain the plant death beyond the tilled circle.
In the snow circles the pattern was clearly spiral, which would be caused by a single tool and cord walked around a center point (post). Without an aerial view, the trenches in these circles appear concentric, which could be explained by the use of modern multi-wheeled ploughs.
Nice to see you open to mundane explanations Lindy :)