Even before the Civil War, the mindset that the North was opposed to slavery and the South was not had begun to set in.
In 1850, Missouri wanted to become a state, but one that allowed slavery. While northern states were tolerant of slavery, they did not want it to spread. Because of that. Missouri was refused admittance in to the Union. Henry Clay, known to many historians as "The Great Compromiser" wrote up The Compromise of 1820, or The Missouri Compromise. One of the components of the compromise stated that all of the territories north of the 36'30'' latitude line would not allow slaves, while all territories below the 36'30'' latitude line would allow slaves.
The issue of slavery was danced around for the entirety of the USA's life up until the election of 1860. The Republican candidate, Abraham Lincoln, campaigned that he would bring the issue of slavery to the table. He would force people to address it and come to a conclusion on what they would do. Northerners felt uncomfortable about it, but it was one of the South's "querks." Many Southerners didn't own slaves, but it was a part of the traditional way of life they were trying to hold on to. Lincoln wanted the South to realize that their way of life had long since died, and that they needed to move on. Slavery would destroy the South economically and if they didn't introduce industry in to the argicultural setting the South had been maintaining, they would fall behind the rest of the country and the rest of the world.
Addressing this issue would mean change, and the South didn't want chance.
Another thing that Lincoln supported was a Protective Tariff. A Protective Tariff is a complicated law that developed over time.
Apparel was produced in England, France, and New England. The apparel from each country, or region of a country, was of approximately the same quality, so all three countries sold their product for the same price. The companies of New England did not like this, and rallied the US government to pass a tax on imported goods. At first, this tax was 10%. Since the cost was higher for the production and transportation of the apparel from Europe, New England companies believed that more people would buy goods from their companies. 10%, they discovered, did not make a large enough difference in the costs between European and New England goods to affect their sales. Eventually, they managed to raise the tax, or Protective Tariff, to 35%.
The South had a problem with this. They were the sole producers of cotton for the entire world. They were able to sell their cotton for large sums of money. However, as the Protective Tariff got higher, European companies, some of the leading buyers of cotton from the South, started to lose too much business to stay in business. That presented New England with a monopoly on the apparel business, meaning they controlled the entire market for cotton. With that control, they were able to lower their asking price for the cotton they were purchasing. That equated to less money for the Southern producers of said cotton.
The South did not want a president who would support high Protective Tariffs, as that was something they viewed to be the source of many of their problems. Since Lincoln supported high Protective Tariffs, he was the last person the South wanted in office.
In December of 1860, it was announced that Abraham Lincoln won the 1860 election. He would be inagurated the following January. South Carolina didn't stick around to see that happen. They seceeded from the union on December 20, 1860. Twelved states followed over the course of the following four months.