While some may say it is a fungus, To me it looks like something was placed in the bed, and it ran down the hill and pooled about 2/3 of the way down the slope

can we rule out dog?

is it the same type of grass... close up pic... if different it may have gone dormant.. while the other type didnt ...

i dont get the layout.. why are there 2 sidewalks so close? ... why do they bend.. why did you need to use a panoramic lens shot? .. dont know that it would make a difference.. but im curious... e,g, is it a duplex.. and your neighbor could have screwed with it???

did anyone clean out the car.. and leave a tarp on the grass all day when it was 105 degrees???

ken

If it is fungus you should be able to smell it. Get your nose down there at soil level or pull up a clump of the brown grass and have a good sniff at the root area.

Where do you live?

What kind of grass do you have?

What is your watering schedule - how often and for how long do you water?