Why did State's Rights become an issue in the 1820's?

Because the Constitutional Convention of 1787 had created a government based on federalism, and the states wanted more power.

The Missouri Compromise in 1820 tried to solve the problem but succeeded only temporarily. (It established lands west of the Mississippi and below latitude 36º30' as slave and north of the line-except Missouri-as free.) Abolitionist groups sprang up in the North, making Southerners feel that their way of life was under attack. A violent slave revolt in 1831 in Virginia, Nat Turner’s Rebellion, forced the South to close ranks against criticism out of fear for their lives. They began to argue that slavery was not only necessary, but in fact, it was a positive good.