The answer to 'shrub thin grass' is that a common cause is the shrubbery blocking sunlight and competing for nutrients with the grass. Symptoms include thinning, patchy grass in the areas around the shrubs. The solution is to properly thin and prune the shrubs to allow more sunlight and airflow to reach the grass. This is typically a DIY task, but you may want to consult a professional landscaper if the shrubs are large or the grass is severely damaged.
Proper shrub thinning and pruning will encourage the grass to thicken up and spread back into the previously shaded areas. Be sure to remove any dead or damaged branches, and thin out the interior of the shrub to allow more light penetration. This may need to be done annually or every few years to maintain a healthy lawn.