It seems silly, but this is actually the confluence of two types of bullshit.

The first has to do with the fear that the federal government is secretly planning to unveil their transformation from "representative democracy' to "totalitarian regime". So to answer your question, Texas would be sorta the first state to fall to the New Nation of Evil Liberal and Possibly Muslim America. Presumably Texas is first because it's such a conservative stronghold. It's armed to the teeth, armed with personal weaponry, private militias, and a huge concentration of American military soldiers. Texas has a huge independent streak, as evinced by the Civil War, by the Independent Texas movement, by their general sense of fuck-y'all that Texas emotes.

The second bit of bullshit is more nuanced, and has to do with the distribution of power between the state's and the federal government. In short, Republicans believe the fundamental unit of governance is the state, that each state is essentially sovereign in itself, and that federal government is a voluntary association between the states that exists to support the states. Democrats, by contrast, believe that the fundamental unit of governance is the individual, and that the federal government is therefore a necessarily unified entity that exists to promote and protect the interests of every individual within the nation. States, then are basically organizational districts designed to make the governance of a huge-ass nation of 400 million people a bit more manageable.

Stuff like "the Federal Government is gonna take over Texas", then, is sort of conservative shorthand denoting that the government is planning to invade Texas by force, dissolve the state of Texas as a unit of governance, making that region directly under federal control (which, to a conservative Texan who believes this, is a horror unto itself), as part of the larger plan to dissolve all the states, for the federal government to directly control every individual in the nation, without representation or consideration. Martial Law nationwide, no habeus corpus, a pure militaristic liberal dictatorial state. Mandatory abortions and gay sex and hijabs from Dallas to Des Moines.

Edit - This is a vast oversimplification of both the relevant conspiracy theories and of the difference in how Dems and Repubs view the state/federal divide, but I tried to capture the essence. There's all sorts of other shit going on too, like how some people expect the Fed to ship trainloads of Americans off to forced labor camps (you'll see stuff in these discussions about "black trains" or whatever), about disarming the populace by force, about genocide of Christians, whatever. Once you've accepted that the government is going Dictator, you can basically believe anything will happen next.