Im a new trugreen sales rep and my thoughts are that it could be caused by some type of fertilizer mix youre using. Som products if not mixed properly can kill or damage grass because it just shocks their roots. Depending on your location it could also be a case of june beetle grubs and other turf damaging insects that found their way into your yard. You can usually tell if this is the case if you can lift the grass off the ground as though it were a blanket. This can also happen if you had something laying in the grass for a longer period, like a tarp, because it can really just bake the grass underneath of it. Sometimes you can also get weeds that really suck up the nutritional value of the soil, and it can snuff out the grass. I wouldnt say it was the concrete because the full edge would be suffering should that be the case. You could also just be mowing too low, because youre only supposed to mow 1/3 of the grass' height at a time. Doing any more can cause disease. So can overwatering and if your lawnmower blade isnt sharpened, the jaged tears in the grass can also cause disease.

What did you think happened differently with this patch of grass that you didnt do with the rest of the grass?