I suspect man-made - as in some operators fooling around or the SyOps boys doodling with a laser from a spy satellite.
Hell, that makes as much sense as anything else I’ve read.
Has anyone done a study to see if the circles occur in areas known satellites pass over? Then again, there’s those satellites we don’t know about. (cough)
LOL! I could also imagine some bored students, most likely from the local engineering/comp. sci./math departments.... heck, I’ve sort of always wanted to do it myself, but somewhere where it’s not actually damaging crops.