The South had been involved in a series of political and economic conflicts with the North for decades. A lot of involved tariffs and trade restrictions imposed on the South by the Federal Congress, at the behest of northern industrial and shipping concerns who wanted to plunder the South's foreign trade lines. Also at issue was whether the States were sovereign and still capable of exerting internal autonomy, the "null and void" crisis.
I've studied the history leading up to the war extensively. I was allowed to take some post-grad-level history courses in college; the two I chose were the Vietnam War and the Civil War, both by the same professor, a brilliant man whose classes on history were always in DEPTH... I mean in the Vietnam war class he started off on Day 1 saying "To understand the roots of the Vietnam war, you have to go back to about 1000 BC..." He was similarly thorough about the Civil War, beginning with pre-Revolution differences between the North and South in climate, economy, politics, settlements, and more, and proceeding to detail the conflicts that lead to the war. He said very plainly that while slavery was AN issue it was far from the only one, and not necessarily the most causal.