The thing people forget when talking about secession is that unless the constitution was explicitly written to make affordances for future secessions, every attempt at secession is illegal and can be considered an act of civil war.
Aside from "voting irregularities" one of the major talking points when the Crimean peninsula voted to secede from Ukraine and be annexed into Russia was that there was no legal basis for them to do this, even if the elections had been undisputed.
Slavery.
Interestingly the way people outside the US are taught about the American Civil War is far more nuanced: state rights, economical reasons, sovereignty, etc.
But I guess "the North declared war on the seceding South because the South was literally worse than Hitler" is a valid interpretation too. Teach the controversy, right?
Sure, they could do that, but it's hard to take any sort of moral high ground or project strength on the world stage when you are forced to militarily occupy your own country against its will.
That said, I doubt any states are realistically going to secede, but at least Texas won't be the only ones constantly going on about it now.