What you are seeing is normal die out of your grass for the year.  Kentucky Blue and Perennial Rye Grass build up a 'thatch' annually.  If you don't like the look of it, you can rent a "dethatcher" and it will remove the dead 'brownish' stuff you are seeing.  The best time to do this is in the fall.  That way your lawn has time in the spring to replenish itself.  After dethatching, make sure you rake up and dispose of all loose debris (or compost it).  It is also a great time after dethatching to overseed your lawn.  The grass seed won't sprout yet this fall, but will first thing in the spring.

Hi Sandra,

I'm sorry but I have to disagree with you. I know what thatch is and we have it in the rest of the yard but this is real thick and the brown grass in the picture goes right to the ground. It's not everywhere, just in certain spots every year. Earlier in the year it's green and real thick.

It is also possible that the thin light brown grass is fine fescue which is often in grass mixes. There is a side by side image here. In a mixed lawn, the various species of grass will go dormant at various times.  I hope this helps. 
 https://www.wxintegrations.com/temperatures-this-week-will-finally-allow-for-grass-seed-application/

Ok. Thanks. I'll check it out. It's kind of like a person's hair afro. Another thing I noticed is it seems to be in the wettest areas 

When grass is overwatered (do you have a sprinkler that runs regardless of rains?), grass sets shallower roots because it finds water at the surface.  Run your sprinkler on manual not Auto during rainy seasons.
Also, short mowing to 2.5 inches as opposed to 4 inches results in shallower roots because roots go deeper when grass is allowed to be taller.  Moral: Set your mower high.
The combination of either or both of these ways of promoting shallow roots is that when there is a dry spell as we had in Minneapolis recently, especially in the shorter days of fall,  the grass can go to thatch easier.  (I know you are talking about the grass-not the thatch).
I hope this helps.  PS: Fescue is a fine grass for Minnesota even though it differs in appearance from the Bluegrass we have been used to.

I always mow 4in or higher except in spring and fall. Yes we have a  sprinkler but hardly used it this year 

Then, I am inclined to think that the thinner blades are fescue.  It is thinner and more bendy than grasses we are used to.  I don't have much to add.
I hope this was helpful.
 

Is fescue native to MN? If I kill it and re-plant grass there, what grass seed should I use? We're in SE MN by Rochester. The soil is pretty good dirt.

There are many articles on University of Minnesota research on fine fescue for lawns in Minnesota here: https://turf.umn.edu/categories/fine-fescue  and here. Generally, it is a highly regarded new arrival.  For you, the first step is positive identification of your lawn.  I would start perhaps with taking a sample to the Arboretum for positive identification.  Alternatively, you might take it to your local Extension office  https://local.extension.umn.edu/local/olmsted   
Best of luck to you.

Thanks Steve. Here's a pic of the backyard.

Lovely.
Bye