The Southern state seceded from the Union because Abraham Lincoln got elected. However, that was only the “last straw” that caused them to secede. Basically, they believed that the North was going to dominate them and destroy what they saw as their way of life (which was based on slavery).
Ever since at least 1820, the North and South had been in conflict to some degree. They had different economic and social systems. The South’s systems revolved around slavery. This meant that the two regions felt like they each had different needs. Many things happened that convinced each side that the other was going to take too much power. These events drove the two sides farther and farther apart.
In 1860, Abraham Lincoln was elected president. The South felt sure that he was an abolitionist. They also noted that he was elected without any Southern votes. To them, this meant that he would govern for the North and would do away with slavery. Because they felt that this was going to happen under Lincoln’s government, they seceded.