Shallow roots from frequent but shallow watering. Stress and dormancy from the drought. It's slowly snapping back in other areas as well. Be vigilant with fall weeds taking over in the thin spots, but wait untill the grass has recovered for a while to spot spray if needed. Was the area recently more shaded, before a tree fell or was removed? This can also happen when there's a sudden sun exposure/microclimate change like that, to where it can take a couple seasons to fully rebound.