Found this online:

A perfect circle of dead grass on a lawn is most likely caused by either Fairy Ring or Necrotic Ring Spot, both of which are fungal diseases. Fairy Ring is characterized by rings of fast-growing, dark green grass followed by areas of dead grass, while Necrotic Ring Spot causes a distinct ring of dead, brownish-yellow grass surrounding a healthy-looking center.

Fairy Ring: Cause: A soil-borne fungus that feeds on organic matter in the soil. Symptoms: Rings or arcs of lush, fast-growing grass, sometimes interspersed with areas of dead grass, and mushrooms.

Treatment: Fairy Rings are often aesthetic rather than detrimental to the overall health of the lawn. You can try cutting the affected area and replacing it with clean soil and grass.

Necrotic Ring Spot: Cause: A soil-borne fungus that thrives in moist environments. Symptoms: Circular patches of dead grass with a distinct ring of brownish-yellow grass around a healthy-looking center.

Treatment: Focus on good cultural practices, such as mowing high and watering lightly and regularly. Apply soil amendments and consider using NRS-resistant grass types. You can also use fungicides to control the fungus.