(CNN) On Monday, Texas Republican Rep. Mac Thornberry announced he is not running for another term in 2020. Which, on its face, is a totally a normal thing! Thornberry is term-limited out as ranking member of the House Armed Services Committee and has already spent 25 years in Congress.

But with his retirement announcement, Thornberry becomes the 6th(!) House Republican in Texas to announce his retirement ahead of the 2020 election -- a remarkable 1/4 of the entire 23-Republican strong delegation. In fact, almost half of all House Republicans who are retiring without seeking another office are in Texas

In each episode of his weekly YouTube show, Chris Cillizza will delve a little deeper into the surreal world of politics. Click to subscribe!

So, why?

Well, part of all of these sorts of decisions are individual. Thornberry didn't want to be in Congress without his high perch on Armed Services. Rep. Bill Flores wanted to go back to the private sector. And so on and so forth.

But none of these decisions were made apart from politics -- and one calculation in particular: The rapidly changing demographic face of the Lone Star State.

Hispanics are now projected to be the largest demographic group in the state by as early as 2022. In 2018 alone, Texas gained nine Hispanics for every new white resident, according to an analysis of the Census numbers by the Texas Tribune . And while Republicans have been struggling to win over Hispanic voters since George W. Bush's 2004 reelection race, President Donald Trump's strong stances on immigration have moved those voters even further from the GOP.

Of the six seats Republicans are vacating in Texas, two are so GOP-friendly as to be immune from the broader trends (non-white voters growing in number, white voters, especially suburban ones) moving away from the party -- at work in the state. Trump won Thornberry's 13th district with 80% in 2016 and took 78% in Conaway's 11th district.

But the other four districts -- held currently by Flores as well as Reps. Pete Olson, Will Hurd and Kenny Marchant -- show the movement away from the Trump Republican Party that had to play a factor in these incumbents' decisions not to run again. Below, what Trump got in these four seats in 2016 compared to Mitt Romney's percentage in them in 2012.

TX-17 (Flores): Trump 56%, Romney 60%

TX-22 (Olson): Trump 52%, Romney 62%

TX-23 (Hurd): Trump 46%, Romney 51%

TX-24 (Marchant): Trump 50%, Romney 60%

It's not surprising, given Trump's fade in that quartet of districts, that three of the four -- all but Flores -- barely eked by in their own reelection races in 2018. Olson and Marchant won with 51% of the vote, while Hurd won with just 49%.

Those results at the district level jibe with the broader competitiveness statewide. Sen. Ted Cruz (R) won reelection, yes, but with just 51% over then-Rep. Beto O'Rourke -- in a race that cost upwards of $115 million. And now, Democrats insist they will make a real run at Texas Sen. John Cornyn (R) next November.

Texas is changing. Quickly. And it is the leading edge of a broader demographic reshaping of the country -- particularly in the South and southwest -- where the rise in the Hispanic voting-age population coupled with the clear Democratic tilt of those voters will make it much, much more difficult for Republicans to hold areas and even states where they were once totally dominant.

In short: The half-dozen GOP retirements in Texas are a sort of early warning sign for Republicans heading into the 2020 election and beyond. They can ignore the alarm if they like. But they do so at their own (electoral) peril.