Slavery was about all. They saw their influence waning when free states started being admitted without a balancing slave state. Iowa was the first. It was the election of Lincoln and the abolitionist Republicans that pushed it over the edge. They didn't have the power to stop them anymore.
http://www.civilwar.org/education/history/primarysources/declarationofcauses.html?referrer=https://www.google.com/Edited by Alferd Packer at 16:26:32 on 05/01/17