I assume you are talking about fairy rings, where mushrooms form a perfect circle on lawns and other area. They grow in that shape because the mushroom is just the fruiting body of a huge organisms that is mostly underground. In simple terms the mushroom is just the "flower" of a plant that exists mostly as a massive underground root system. The part of the organism that is underground is circular because it grew from a single spore, and then spread out from their. Just as the canopy of tree is circular so the hyphal mass of a fungus is circular. When conditions are right the fungus produces mushrooms on the youngest growth, and because it is round the youngest growth exists at the periphery of the circle. The result is fairy rings, circles of mushrooms.