Obviously they did no such thing. They exercised their sovereign right to "resume the powers of government" they had delegated to the federal government after determining the federal government had become abusive of their rights and happiness. Obviously preservation of slavery was not their big concern because A) slavery simply was not threatened in the US and B) lest anybody think it was, the North was only too happy to offer express protection effectively forever via the Corwin Amendment....something which the original 7 seceding states turned down.
The Upper South only seceded when Lincoln chose to start a war. They had been content to remain in until that point.
Listen Reb: The South seceded to preserve slavery, opened fire on Ft. Sumter and ended up losing the wr it started.
You Johnny Rebs are trying to re-fight 1865 and it's not going to happen YOU LOST! Grow up!