Protection of NE industry by tariff which was retaliated by European tariff on agricultural products.
Money talks. Did then too.
Tariffs were cited by the seceding states, but subordinate or attendant to slavery. And with the south dominating both Congress and the White House for most of the years preceding the Civil War the best that could be said of it (or the worst against it) was that this was all part of the give & take between states and regions of the country and to say that tariffs were responsible for the insurrection would be tantamount to saying that they rebelled because they didn’t like the drapes.
Slavery, especially in the context of expansion into the territories was at the heart of the souths reason to rebel.