I am just guessing here as is everyone else. I really think it appears to be drought stress or insect related. I am leaning against it being over fertilizing just based off of the photos because the turf does not appear that it is really dark green for a SA variety but maybe it is just the photo. Also if it was just a case of a little fert being over applied in those areas I would think the areas which have not declined, well the areas right outside of the declined areas would be super dark and thicker because of the excess nutrient super charging it. Another sign I would assume would be there would be GLS on the leaf blades because over fert, over water and our environment usually lends itself to GLS from my experiences. Les a couple of questions - 1. are you familiar in scouting for chinch bugs and if you are have you looked in the green areas right outside of the declined areas for them? 2. I know it is obvious but have you check your irrigation system to ensure it is operating properly and heads are not clogged/or rotating properly to give good coverage.