In a nutshell an encroaching usurping Federal Government. You cannot judge the republic of then by today's standard. Back in the day, the States were supreme. Hell, the smallest thing FedGov™ tried to do was considered encroachment on States Rights then and rightly so. We are so lost now, nobody can even understand what it was like, the glory days of the republic.
The only usurping that the Southern states were concerned with was the fear that Lincoln would prohibit slavery in the territories. Doing away with slavery would have eliminated that problem so it's back to the original question - had the Southern states done away with slavery then what was there to secede over?