This spring there was a lots of dead matted grass in the yard (see pictures 1 and 2). So I manually dethatched the lawn with a dethatching rake in late April. I removed several bags of dead grass. Then a week I used an electric dethatcher at the highest setting and removed many more bags of grass. A week or so later I fertilized the lawn. See picture 3 for the way the lawn looks today. I noticed that several lawns in the area have a similar issue. What may have caused the dead grass and what can I do about it? Thanks.