In my day, they were just dudes doing brodies in their crap-box Chevy Tahoe out in a farmers field. The alien thing was a quick excuse for the circles left in a guy's field. It didn't explain the beer cans and other detritus left behind so I don't know how it caught on. Now if someone could explain cattle and wild horses dyed pink around where I grew up.......
It would be funny if there were no grand design to communicate through symbology.
Maybe it is just alien adolescents out joy-riding.
It turns out their xenobiology lets them get high off cow farts like cats get high off catnip. When the adult aliens aren't watching they sneak down to the rural surface spot at night and snatch some cattle. Hover around in the dark and stick their tenacles up the cows kazoo and snort.
Eventually, as youngsters will do, one will say, "Hey...ggigle, watch this..." and zooms down over a fields and stamps out a circle in the middle of a field to freak out the surface monkeys. Like trenching a yard.
They crack up a while and another says, "Hold my cow..." and drops down and stamps out a more complicated one. "Let them grok on that a while. Lolz!"
That's how it starts. Sure all in good fun.
Kids.