Why did the southern states decide to secede?

The North was becoming increasingly abolitionist, which would have eliminated the one area (agricultural production) in which the South had an economic advantage. When the Republicans, and Lincoln, came to power in the 1860 elections, the South knew that they faced both increased taxation and the end of slavery, and decided to form their own country. This, obviously, was unacceptable to the federal government and war would soon erupt.