So far as slavery being the cause of secession, the fact is many men in the South including Jefferson Davis knew that secession would be the doom of slavery. 

Slavery could not be economically viable or legally enforceable where freedom was just a river away.

They had pushed the North so hard to enforce the Fugitive Slave Laws for just this reason. Alexander Stephens was among those who judged "slavery much more secure IN THE UNION THAN OUT OF IT."