No matter what anyone tells you, thatch and clippings on your lawn are not one and the same — and, no, clippings do not cause thatch.
Grass clippings, the portion of the mown grass, are about 90% water, so they begin to decompose almost immediately after hitting the ground. Left in place, clippings return nutrients to the soil. Lawn thatch, on the other hand, is the dead grass and root tissue between the green vegetation and the soil surface. In layers of 1/2 inch or thicker, thatch blocks water, air and nutrients from reaching the roots and provides a nesting place for insects and disease.