I was looking at House results for the 2020 election, and I found one thing that stands out to me. When comparing the 2020 House elections to the 2020 presidential election in South Texas, the shift towards Republicans was concentrated in the race for president. In addition, Zapata County, an almost totally Hispanic county Trump narrowly won, Cornyn and the House Republican lost.
It is kind of surprising to me considering the fact that this is a mostly Hispanic border region that such a shift didn't particularly carry over to the same degree.
There were many reasons South Texas switched toward Trump, but not necessarily down-ticket.
Biden did a lousy job at Hispanic outreach, particularly so in South Florida and South Texas. Hispanics are not a monolithic voting block, but his campaign treated them as such.
Hispanics from South Florida are much more likely to be refugees / descendants of refugees from communist / communist-leaning countries than are Hispanics elsewhere. Whether valid or not, attacks on Biden verging toward socialism took hold in South Florida, particularly in Miami-Dade.
Hispanics in South Texas don't see themselves as Hispanic so much as they see themselves as Tejanos, and see themselves as very distinct from the more recent Hispanic immigrants to Houston and Dallas.
South Texas voters tend to be
Strongly pro-oil. A lot of people in South Texan work in the oil industry. Biden stating that (source) "I would transition from the oil industry, yes, because the oil industry pollutes significantly" in the last Presidential debate did not help Biden's campaign in South Texas.
Strongly pro-border control. The US government, and border security in particular, is another big employer in South Texas.
Very strongly pro-2nd amendment. Towns in South Texas become near ghost towns on fall weekends because so many of the citizens are out hunting.
Very strongly anti-abortion. Catholicism is the number one religion in South Texas.
What about down ticket? I can see several reasons for this as well.
Texas eliminated straight ticket voting in 2020. (source) The Texas state legislature voted in 2017 to eliminate the straight-ticket voting option starting in 2020. (HB25)
Split-ticket voting has been a long-standing tradition amongst some across America.
While Texas in general switched toward voting Republican more than two decades ago (Ann Richards was the last Democratic governor of Texas; her term ended in January 1995), South Texas remained a Democratic stronghold until recently.