What was the deciding factor that cause the first group of Southern states to secede from the Union?

What was the deciding factor that cause the first group of Southern states to secede from the Union?
The first states seceded when Abraham Lincoln was elected president. Officially, Abraham Lincoln believed that all new states admitted to the Union should be free states. Although Lincoln was viewed as a moderate in the North, he was considered an extremist in the South. Southerners feared that Lincoln would not allow slavery to continue at all, even though Lincoln did not espouse this as his official position.

South Carolina was the first state to secede, declaring itself independent from the Union.