The most immediate cause of secession was the election of Abraham Lincoln on a platform that Southerners expected to be an engine for ruining the South economically, and permanently changing the relationship of the federal government to the States, in order to cement and entrench Lincoln's factional victory.
Southerners feared that Lincoln, in possession of the federal government, would use the Supremacy Clause to abrogate the Ninth and Tenth Amendments, and the rights of the States. Which is about what has happened.
If you look at Non-Sequitur's long post of quotations from Southern secession speeches to you, you will see those concerns reflected there, as much as, or even more than, any solicitude for slavery per se, or even its economic ramifications.
Remember that Marxists are determinists, and economic determinists in particular. That is one reason that pushing the line that "it was about slavery and nothing else" comes so easily to them. It is convenient to their political purpose of building a Marxist superstate, and consonant with their own indoctrination by senior Marxists.
Since to Marx all struggles were economic in nature, and all institutions of a culture reflected the imperfect class structure of that society, it is only natural that Marxists would define the Civil War's roots as class struggle writ large. And of course, the most graphic illustrations of class struggle are the antipodal institutions of aristocracy and slavery, those institutions that historically defined the antebellum South.
The Marxists are not WRONG in their view, simply tunnel-visioned. Their narrow definition fails to appreciate the enormity of the clash, and understates the ideological fault line that has run through our "union" ever since its conception.