They are caused by fungi, nemitodes or insects in all cases as I remember. Grubs and nemitodes can eat around in circles or rings below the surface and lawn moths can leave circles above ground.
You need to treat with appropriate insecticide and fungicide products. I have been out of turfgrass management for a long time but suspect you need permits and applicator licenses for products that will actually do much. They are not available over the counter to consumers. A call to a lawn service company might be a cheaper alternative long term. If your region or state still has an ag extension office with any budget left to have someone answering a phone line you might call them for recommendations.
In general things you can do to fight back--especially after you resolve current outbreaks? Regular feeding of turf grass is always the best defense against anything trying to get at it. Water deeper, for a long period of time, but less frequently once your lawn is established. And try to water later in the day so the turf is dry by sunset. Bag your clippings if you suspect a fungus outbreak so you do not spread it.
A forum community dedicated to Do it yourself-ers and home improvement enthusiasts. Come join the discussion about tools, projects, builds, styles, scales, reviews, accessories, classifieds, and more! Helping You to Do It Yourself!