It was primarily about the right of secession.
Except they wanted to secede because of slavery. It seems that Bill Clinton didn't invent such parsing -- southern slave holders and their modern day supporters seem to be well versed in the technique as well.
"Oh, oh, we're just fighting for the right to secede."
"Why do you want to secede?"
"Oh, just because -- nothing to do with slavery, nope, no sir."
The south seceded because of the election of Lincoln, which, to the south, foretold how the issue of slavery was to be dealt with. So (in legal analogy) Lincoln's election was the proximate cause, not slavery, although slavery was an actual cause.