How come mushrooms sometimes grow in a big circle?

Question

This keeps happening in our yard, even when we pick them or mow them down. They grow back in the same spot, and they form a big circle. It's really weird.

Answer 1

But the circle is slowly expanding, doesn't it?

Those mushrooms are actually not individual mushrooms, but just the above the surface part of one large underground mushroom (the underground part is mycelium, that looks like a fine mesh of filaments). The mushroom expands just like the ring of a tree.

Answer 2

If the ground is of uniform character and nowhere hinders the propagation of the subterranean vegetable in one direction rather than in another, the mycelium  ( a mushroom is the blossom of a plant that lives under ground and is called  mycelium ) spreads equally on all sides, and so produces circular groups of mushrooms, which the country people sometimes call  ''witches' circles."