Can you get a closer picture of the spots? Could be insect damage. Pull on the brown spots and see if it comes up easy or is still rooted.

 

Get a 5 gallon bucket and fill it up with water and a little dish soap and pour it in a spot close to the damage, wait about 5 minutes and see if any bugs come up. I had a similar issue and it turned out to be mole crickets that I am fighting now. I sprayed some Bifen XTS and that night my wife was walking out the side door and told me I had to see this. I laughed figuring it was a raccoon or something because she seemed really excited and it was two mole crickets dead right next to the door which is a good 30ft from any grass. Now I don't know if the lawn gods put them there or it was the cat, but I found it pretty humerous.

 

I too had mole crickets that caused similar looking damage. If you pull on the dead grass, it will come right up because the roots have been eaten. I believe grub damage looks similar. Webworms will eat all the green but the grass will still be rooted. Bifen XTS took care of my mole cricket issues (and some minor webworm issues as well). I was finding lots of them in the pool after treating. Some were huge so they had been around for a while. Next season I'm going to get on a monthly treatment schedule at a lower maintenance rate.

 

I was able to get some closer pictures. The grass definitely pulls up easy in those spots. I will try the water and dish soap trick tomorrow and see what happens.