Michael, who was an archangel, slowly drifting on a warm sunny day above the Earth, saw Tommy playing by the side of a pond. The little boy was throwing small pebbles into the water, producing ripples which spread in wider and wider circles from the point of impact. Being of an aesthetical mind, Michael was impressed by the simplicity and beauty of the wave patterns. Sometimes Tommy would manage to make a pebble bounce once or twice on the water surface, causing very interesting interference patterns to appear. Michael would like to be able to do it himself, but, being a pure spirit, he couldn't hold anything, including stones. And he went back to Heaven, slightly disappointed. (This "went back" should of course be read as a metaphorical mode of expression). This happened about the time William Shakespeare was writing his plays.
Emmanuel was an angel, therefore one step below Michael in the celestial hierarchy, but they were good friends. After learning from Michael the contents of the previous paragraph, he thought and thought about it. Being more scientifically inclined, he soon devised a satisfactory alternative to the "pebble throwing into the water" game.
It just happens that these creatures' halos are very strong energy reservoirs. Emmanuel also preferred permanent to transient art forms, and about three centuries later (time is not that important in angelic matters), after doing some experiments on his own, he approached Michael for a demonstration of his "halo throwing into the field" game. He had tried several types of fields, but he had found wheat fields to be the most appropriate, because individual plants possessed a very strong angelic susceptibility. So they would throw their halos to the wheat field (again "throw" shouldn't be taken too literally) and, although in the beginning they could only produce circles, with a bit of practice, varying the way the halos were projected (spinning, tumbling, rolling...) other kinds of formations started to be generated and wonderful patterns were formed on the field. One of the properties of the halos being their boomerangness, they would bounce back to the owner and the game could go on.
Now all this went on very quietly while there were only two or three players. But when word spread about the fun of it, every possible winged spiritual creature wanted to have a go at "halo-throwing", absolutely unaware of the commotion they were causing on the Earth, where circles and other figures multiplied in the fields as if a gigantic geometry lesson had been under way.
At that time St. Peter showed his presence in the playing field, and told the angels: You have been originating a lot of discussion down there, diverting people from their duties, leading them to the production of crazy speculations. I spoke to the Master and he said: STOP IT!
And from that moment onwards, no more crop circles appeared.
All the angels were very upset, and Heaven being a place where nothing happens (well, most of the time) they went back to super-novae explosion spotting, which is a bit less demanding activity than watching black holes swallowing surrounding stars.