Why does my lawn thin so badly?

So i ask this believing that i already know the answer but would like to throw it out there anyways just to see.

This is what my lawn looked like in the Spring



And this is what it looks like now



First, living in Oregon we have very wet 40-50F degree winters, which is why the ground looks soggy after days and days of rain.

Second, ignore the poa popping through, you can see i sprayed it with Tenacity as a test. I didnt get any pre-emergent down due to timing with other life things going on at the time. I dont believe it is playing a part in other sections of the lawn thinning out so extreme.

This is the original lawn that was planted in '99 before i bought the house, and was only moderately maintained by the previous owner to keep it green and not much else. So a lot of the 'grass' is probably old contractor cheap varieties mixed in with the overseed i did with newer varieties in Spring.

The first is an image after being overseeded in Spring with a Rye/Fine Fescue blend.
Obviously the second image from today looks like crap. It was never diseased, or even browned, it just "disappeared" over the course of the last month of rains.

My thought is that in the first image a large portion of the grass that i am seeing is actual weed grass? Or fine fescue that has now died off for whatever reason?

My plan is to nuke this entire section(and the street strips) this fall and replant some ***/Rye blend, similar to what i did in my back yard this year that still looks much healthier.

I guess i just want to hear feedback on what causes this, so that i can work to ensure it doesnt happen again when i do reseed.