That was interesting. I'd never quite thought of it that way before but it does make sense. The slave states tried to get the federal government to force the citizens of non slave states to protect southern slavery and to even actively hunt down runaway slaves in their areas and turn them over to slave bounty hunters and slave owners. So the non slave states started passing state anti-slave laws to protect their citizens from an unconstitutional law. This made the South angry and since they couldn't get the federal government to enforce the Fugitive Slave Act, they chose to secede from the union instead, just to protect slavery and proving once again that the real cause of the Civil War was slavery.
This “Confederately correct” orthodoxy that the South fought for independence, not slavery, rankled a few southern realists, including the editors of the Richmond-based Southern Punch in 1864:
” ‘The people of the South,’ says a contemporary, ‘are not fighting for slavery but for independence.’ Let us look into this matter. It is an easy task, we think, to show up this new-fangled heresy — a heresy calculated to do us no good, for it cannot deceive foreign statesmen nor peoples, nor mislead any one here nor in Yankeeland. . .
Our doctrine is this: WE ARE FIGHTING FOR INDEPENDENCE THAT OUR GREAT AND NECESSARY DOMESTIC INSTITUTION OF SLAVERY SHALL BE PRESERVED, and for the preservation of other institutions of which slavery is the groundwork.”
How people convince themselves that the Confederate flag represents freedom, not slavery - The Washington Post
That theme ran thick in just about all the secessionist advocates speeches, documents, newspaper columns and sermons.
Well before the war. In the 1856 election, when the first Republican ran, the papers were talking about how the division (over slavery) was sure to come and how "the conservative South (soon) will burst forth in a carnival of blood..." - 1856.
They'd been banging the drums of secession and war if anything disturbed their precious little "peculiar institution" for quite a lot time...