A man supplied excess fertilizer and water the grass well. After sometime the leaves turn brown because

A man supplied excess fertilizer and watered the grass well. After some time the leaves turn brown because osmosis occurs in root and root dies.
Too much fertilizer can burn or desiccate roots, ultimately killing the plant. Too much fertilizer can also be harmful to the environment.
Over-fertilization of commercial pots or lawn plants results in high concentrations of soluble salts in the potting medium. These salts damage roots by slowing the net flow of water into the roots and indirectly by predisposing the plants to certain root diseases and damping off them.