Some states, such as arkansas, joined the confederacy only after the attack at fort sumter because they

Convinced Lincoln would suppress states' rights--Lincoln's response to Fort Sumter convinced many states that the federal government had become corrupt and could not be trusted. 

Lincoln responded to the siege of Fort Sumter with military force showed many southern states that the government was overstepping their power. States were willing to fight the "corrupt" government and follow South Carolina's lead to separate themselves from a government that became too large and restrictive.