Why Did they South Declare SECESSION from the USA?

It was a states rights issue, mostly to do with tariffs and slavery. The South felt that the North was denying their constitutional states rights by enacting things that benefited the North and hurt the South, and then got mad at them when they tried to enact the Nullification Act which would have been a major victory for states rights. When Lincoln was elected president, they felt that the end was near, and they seceded.