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: I was wrong. The letters are almost 100% "bla, bla, bla, we're being treated unfairly and we're leaving." Surprisingly little mention of slavery, but get a load of Mississippi's letter! LOL my god, y'all just gonna have to read that one yourself. (I had always assumed that letter was typical and I was wrong.)

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.