Texan here, It is a combo of both. The problem is the people running from blue states bring their families (Multiple voters) whos kids will grow up to vote blue most likely because there parents will only let them hang out with other liberals. The liberals tend to be the loudest and will gang up on school boards that do not do what they want, threatening them with being 'cancelled'. This leads to many schools in major cities becoming brainwashing hubs. Talk to the youth of small towns and cities and most are very conservative. Most of the Mexicans from small towns also tend to be quite conservative, its the Mexicans in the major cities that tend to be Libtarted.
TL;DR It's a combo of both Immigration from blue states to the major cities and forcing schools within the major cities into indoctrinating the youth.
(sorry for grammar as I am rather drunk)
Edit: This is not a new thing, and has been happening for a long time, the Major cities have gone past the tipping point and are even run by liberals now, so the possess is much faster there. The San Antonio of 50 years ago is far different than the San Antonio of today. the same goes for Austin, Dallas, and Houston. The reason you see many border counties vote blue is a combo of Illegal votes and Cartels.
The depth of the Cartels in Texas is far beyond what most people think, They run entire cities. They are not openly violent in Texas as they are in Mexico because it brings to much attention. They own/take over HUGE chunks of land along the border. My family has been in Texas since the original 300, I am Texan to the core. The Cartel has a disgustingly large grasp on Texas that goes far beyond border towns, They own many large Real Estate ventures in the Major cities from Apartment complex's to Corporate buildings via proxy companies.
Texan here, It is a combo of both. The problem is the people running from blue states bring their families (Multiple voters) whos kids will grow up to vote blue most likely because there parents will only let them hang out with other liberals. The liberals tend to be the loudest and will gang up on school boards that do not do what they want, threatening them with being 'cancelled'. This leads to many schools in major cities becoming brainwashing hubs. Talk to the youth of small towns and cities and most are very conservative. Most of the Mexicans from small towns also tend to be quite conservative, its the Mexicans in the major cities that tend to be Libtarted.
TL;DR It's a combo of both Immigration from blue states to the major cities and forcing schools within the major cities into indoctrinating the youth.
(sorry for grammar as I am rather drunk)