Someone has obviously never read the
Declaration of the Immediate Causes Which Induce and Justify the Secession of South Carolina from the Federal Union,
nor A Declaration of the Immediate Causes which Induce and Justify the Secession of the State of Mississippi from the Federal Union,
and of course they obviously aren't familiar with A Declaration of the Causes Which Impel the State of Texas to Secede from the Federal Union.

Because they make very clear that slavery was the heart of the matter under contention.

But please tell me how you know better than they did as to why they were leaving.