University students. The brainy type who like to play tricks on people. Quite a few turned up in the fields within an hours drive of Oxford and Cambridge. (Brainy students with good farmland nearby.)
They sneak out in the middle of the night with boards and scythes, carefully walk to the middle of the circle and work outwards using a stake and rope to give the radius of the circle. Then they walk back out in single file carefully putting the stalks back up if they have trodden on any.
In Devon there is a stone circle of nineteen stones about 30 metres across in a field near Seaton - all the stones are about the height of a small man and weigh about half a ton. It looks authentic but it was put there by a "character" farmer in the 1980's with his digger and tractor. After a year or so the soil bedded down and the grass grew and if you stumble across it you'd think it was "real" at first glance.
Edited to add - Just saw punkofnice's video - sorry punky, I should have watched it first!