Agrarian South vs. Industrialized North made for an unfair trade balance. You can hardly trade a bale of cotton for a steam engine, that kinda idea I believe. Been 30+ since college American History, forgot the exact gripes.
We could probably find these disputes in the various Letters of Secession. They almost all start with slavery, but there were other complaints.
EDIT: Oh fuck me, I’m wrong again. The linked are merely the official ordinances, not Letters of Succession. Hence why they’re all dry legalese. But I did arouse your curiosity about Mississippi, so here go their letter.