Baffling. Unusually straight sides. Almost like a tarp was laid on the grass and a bit of extra water ran off to the sides.
Or are there sprinkler spray heads for irrigation and those zones were not working?
Check for soil moisture levels in the brown and green areas.
Or...it could be an unusually inept fertilizer spray burn. Not likely. Check for signs of dead weeds.
Also check for the existence of a septic tank or other underground buried utilities or trash.
Can you get down 18 inches with a soil probe?
I have a customer who had a rectangle smaller than this and the soil probe would go about 4 " than stop at an old concrete patio, that someone put soil on and has grass growing on it, but not very well.
Was there a table and chairs set up?
Wedding or reception with free champagne or drinks at a table or temporary bar?
Free aeration due to the high heels. Check for that.
Drill team or band practice?
Check for compaction due to foot traffic.
Sprayed with something? Probably not--unusually uniform. Roundup?
Was there a pile of lumber or drywall during construction?
Solar pool cover?