We do understand it, from their own words. They were losing power to the increasing number of free states entering the union. By the election of Lincoln, the south had lost the ability to control the government and protect slavery. Slavery was embattled and the south was losing footing. With the election of Lincoln without a single slave holding southern state, they could see the end was near.
Secession was meant to preserve slavery. After 300 years of the south fighting to maintain slavery, it seceded when the tide began to turn. Within the blink of an historic eye, just five years from the south pulling out of the union, slavery was abolished. Clearly only the south managed to maintain the institution of slavery.
Slavery was abolished in the United States. It continued elsewhere. Saudi Arabia outlawed slavery as recently as 1965.
Slavery was abolished in the blink of an eye, true, but at a tremendous cost. It could have been abolished at a lengthier time scale without the military intervention of the federal government and the subsequent distortion of certain constitutional protections that we lost because of it. On the other hand, some slaves gained their civil freedom and to some people that end justified the means of achieving it.