Have you looked around your lawn recently? Have you noticed yellow or brown patches that are beginning to spread and get larger?
If so you may have a disease in your lawn called take-all root rot. It's caused by a soil-borne fungus which is typically found around turf grass roots.
This fungus will typically cause little damage to the turf unless it goes unchecked. But when it goes unchecked, it can be destructive. Its effects aren't generaly visible to the naked eye above ground until the root system is severely compromised.