Lack of Water
Lack or insufficient application of water can cause the lawn to experience drought within days and begin to brown. Uneven or spots of browning can be attributed to differences in thatch thickness, uneven distribution of moisture in the underlying soil or more intense sun in certain spots.
Chemical fertilizers when applied in excess or over dry lawn grass can cause a burn that kills the grass tops and roots. Fertilizing in bright sunshine of mid-day or in the heat of summer when the grass is already under some stress can also exacerbate brown spot damage from fertilizer. Always apply lawn fertilizer over moist soil in the recommended dose erring on the side of less not more product. Watering in fertilizer after application help rinse the product off of the grass blades and percolate it into the soil where it can feed the roots.
Mowing your lawn too short for the needs of the specific grass varietal removes the ability of the grass to photosynthesize and strangles nutrient development within the blades. Scalping the lawn too low or removing too much blade length in any one setting can cause shock. This will quickly lead to browning in spots or swaths. Never remove more than one third of the blade length in any one mowing session and keep your grass in the preferred blade length range for its type to prevent this from occurring.
Animal urine, particularly repeated deposits from dogs, can quickly cause brown spots in a lawn. Urine spots baked in hot sun can quickly dehydrate and kill grass in spots. The urine is high in salts and nitrogen that are harmless is they are diluted and dispersed with watering, but problematic if left alone.
The placement of barbecue grills, garden hoses, bird baths, kiddie pools and the like over growing grass chokes off sunlight, crushes the grass blades and compacts the thatch. Items left on grass for even a few days will cause photosynthesis to cease, disrupt the flow of water and nutrients through the grass and begin the browning process. If the grass is not killed and the items are removed within a few days on occasion the grass can be brought back to life. After extended periods the grass will die and need to stripped and reseeded to blend into the surrounding grass.