See how the ground slopes downward to the retaining walls in this photo from last summer? A little too much slope... not even... kind of a fall hazard. And the grass didn't grow up to the retaining wall, leaving this dirt space (possible encouraged by Mike and his trusty Roundup).
This spring my folks hauled in about half a dump truck of topsoil, and tried to even out the grass surface. They then seeded it, and overseeded the rest of the lawn in an effort to even it out.