If they had freed their slaves then what would they have had to secede over?
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.