This is the popular theory, but more accurately stated. Many people say it's all about the "expansion" of slavery, but that claim doesn't pass cursory examination.
I have long said that the reasons for the South wanting to leave are irrelevant to their right to leave for any reason. The only salient point here is whether or not an association voluntarily joined can be voluntarily left.
The Declaration of Independence says "yes", and the Constitution is silent on the matter. The concept of "Freedom of Association" requires the corollary of "Freedom of Disassociation", and so by the base foundation of natural law, people cannot be required to associate with people whom they do not wish to associate.
Whatever were the South's reasons for wanting independence, they had a right to have it, and so I see efforts to focus on their reasons as attempts to deliberately distract from the core principle involved.
"Consent of the governed."
Do you really think that the deep South states would have seceded if Breckenridge had been elected President.
It's pretty ironic to make that argument in the defense of slaveowners.