The South seceded from the Union in two waves. The first group of southern states seceded after Abraham Lincoln was elected President. Seven states seceded at this time. They were concerned that President Lincoln would end slavery, even though he never said that he would do that.
The second group of southern states seceded after the attack at Fort Sumter. Four additional states left at this time. These states left because they had concerns about what would happen to slavery now that the fighting had begun. They waited until the Civil War began before leaving the Union. These states were convinced after the battle at Fort Sumter that their needs would no longer be met in the Union. They feared that slavery would end once the Civil War had begun. They were worried about what would happen to the southern way of life if slavery ended. They knew that joining the Confederacy was the best way to protect slavery and the southern way of life.
The southern states that seceded after the attack at Fort Sumter did so to protect everything that they had known for so many years.