Just guesswork - not even a theory - but I wonder if they could be some kind of acoustic phenomenon, like the patterns that will appear in sand on a flat surface when a violin bow is played across its edge, or water patterns in a Tibetan bowl? I'm not sure they really look like that, but maybe it should be considered.
One oddity that I don't think has been mentioned yet: are there any reports of poorly formed circles, as you'd expect when people are learning to make them with planks and so on? You'd expect some practice ones from people new to doing it.
There were a couple of really iffy ones near Bramber a few years back....not much media interest...until the following year when there were two more defined patterns. Another question springs to mind...Are these sites "governed" by magnetic earth pulses/cracks in ley line paths/ease of viewing for all and sundry/difficulty in viewing whilst being made/closest to a pub/none of the above? Each possible solution has it's own set of questions.