Here's the thing ... this wasn't a big subject in my Canucki schooling. My first real exposure to it was when I drove down to Atlanta and stayed at a camp ground at Stone Mountain. What I got from the local people was state authority. I'm assuming they'd have more of a handle on the details as their grandparents were the ones doing it .. and not just absorbing what the victors wrote in a history book.
My opinion, they seceded over states rights and the north (Lincoln, et al) knew there was a lot of sentiment for abolition, so they used that as an excuse to go to war. The result, abolition and more federal authority.