THE UFO CONNECTION
If we take the word UFO literally (unidentified flying object), it is obvious that there is a UFO connection. After all, the light balls are still more or less unidentified and unknown. But what about the flying saucers and indeed, the humanoids? Are they making the Circles?
If you ask me, no. There are no clear signs pointing to a conclusion that physical alien vehicles or humanoids are responsible for the Circles (though some circle or triangle markings might be UFO landing marks).
But another issue that is very relevant here, is that most of the UFO sigthings concern the so-called "nocturnal lights" i.e. mysterious lights flying on the night sky. I think it's logical to assume that many or even MOST of these balls are the same that are used to make the Crop Circles.
These lights are often reported to be moving extremely fast and making rapid alterations of direction, apparently breaking the known laws of physics - but only if we assume these balls ARE physical! If they are non-physical energy packets the movements are much easier to understand. If these electrically charged plasma balls are released to the air they might make some "impressive" movements simply because they are very unstable. Another thing that is often reported is the sudden disappearing of the lights. A plasma ball - like the ball lightning, though it's much more unstable - vanishes by itself after some time.
Also if we accept that the balls are actually very small, the speed of the nocturnal UFOs slows down! The estimations of tremendous speed are based on the assumption that the object is far, which is based on the assumption that the object is big. And what is this assumption based on? Nothing else than imagination and predetermination. For it is impossible to guess the size of an unknown object flying on the night sky! It could be 15 meters away, or 15 light years away (unless it flies in front of clouds, of course)!