What was one reason that the Southerners gave for seceding?

Because their agricultural way of life based on large cotton or tobacco plantations was very different to the city based industrial life of the northern states there was a cultural divide. The northern states , being more numerous , more highly populated and more industrialised, started to dominate the relatively new country and so 7 southern states, in line with their constitutional rights for self government ("government by the people, for the people and so on..."), decided to go it alone. Not every southerner agreed with this. The leaders of the 7 cotton states however expected no problem in being recognised as a new country by Europe (or by the northern states for that matter) and no problem in standing on their own feet economically, militarily or politically. They never expected that their desire for self-determination would be so ruthlessly trampled into the dust by the northern bully boys like Lincoln. Other states like Virginia then unwisely joined them after seeing the north's reaction.