Compacted Soil: Compacted soil or heavy clay forces grass roots to migrate up to the soil surface for air where they form a thick mat which becomes thatch when they die.
Too Few Soil Organisms: Soils that are low in organic matter and/or high in clay content or acidity will not have enough earthworms and decomposer microorganisms available to break down naturally accumulating thatch.
Too Much Fast-acting Fertilizer: Fast-acting fertilizers tend to acidify soil. Acidic soil (pH of 6.5 or below) repels earthworms and reduces soil microbial activity as well. This retards thatch breakdown.
Overuse of Pesticides: Certain pesticides may cause thatch buildup by suppressing soil organisms, especially earthworms. Some fungicides may stimulate growth of grass roots and rootstocks, thus increasing thatch formation.
Type of grass: Some grasses produce more stem tissue than others because they spread by runners, or stolons, which are more likely to build up a thick thatch layer. Cool-season examples are creeping red fescue, Kentucky bluegrass, and creeping bentgrass. Warm-season examples are Bermuda, zoysia, and St. Augustine grasses.