Hi Larry,

I'm sorry to hear about your lawn!

You could have a few things (or combination of things) going on.

The first thing to evaluate is to make sure that the lawn is receiving water evenly.  Most lawn problems in Colorado have water involved at some point, and usually we can be in good control of it since it rains so infrequently--so it's a natural place to start.  If your irrigation system is older, or if any heads are not quite vertical, you could be applying significantly less water to the brown area.  Luckily, it's easy to rule this in or out as a possibility by following the steps here: https://extension.colostate.edu/resource/lawn-irrigation-audit-tutorial/

The pine trees adjacent to the lawn could be playing a part too, both by shading the lawn and by competing with the lawn for water.  You might try giving a bit extra water to that section to accommodate for what the trees are using, and consider overseeding that area with fine fescue, which is extremely shade tolerant (though not a perfect textural match for Kentucky bluegrass, it's pretty close, and often included in lawn seed mixes).  

The other possibility which could be contributing is rabbit damage.  Look at the brown area closely for rabbit pellets and for closely clipped green grass (much shorter than the mower would ever get) to see if this could be part of the problem.  If you find evidence of rabbits, the gold standard is "exclusion," if possible.  Look for holes in the fence, etc, where they could be getting in and sealing them. If exclusion is not possible, modifying the habitat nearby to eliminate ready cover, and some repellents can work with some rabbits, but they are far from perfect solutions.

I hope this is helpful; let me know what you find and if you have additional questions.

Cordially,

John