Why do mushrooms always seem to grow...

Not all mushrooms grow in a circle, just the fruiting bodies of a certain type. The spawn starts from a spore falling in a favorable spot. Then a network of fine, tubular threads grows out from the spore in all directions. Thus a circular mat of underground threads is formed. Gradually, the spawn at the center of the circle dies out. These circles are called fairy rings, and they can persist year after year, with the diameter gradually increasing and segments dying out until the ring shape can no longer be discerned.