The Union being pulled apart
The states used to be like little nations. The Constitution limited only Federal power and the Bill of Rights did not apply to state legislatures.
After the Civil War the USA gradually became a fairly unified whole, laws and practices and procedures became much more common among the states and the gradual incorporation of the 14th amendment meant you had the same rights everywhere. (On paper)
It appears, however, that we are returning to the point where a person might move from state A to state B simply to be in state B. No because friends or family or jobs are there, but because of the differences in the state governments.
Is pot decriminalized, or do they throw away the key? Can you get an abortion in the state? Is health coverage easy to get or hard to get? Which versions of biology and history will your children be taught? Does the state welcome immigrants? Can you gamble there? Do you need auto insurance? A motorcycle helmet? Are union rights protected? Is nude dancing allowed? Is a pack of cigarettes $4 or $9? Do you need your birth certificate to vote?
Take all the (expanding) variables and we start to see states so different in legal and philosophical character that they represent very different places, and some people will move to follow their values, exacerbating the differences.