I'm a guy that was born and raised in the South (well born in Missouri, raised in Arkansas since I was one year old and went to undergrad/am going to Grad school in Arkansas). As such, I was taught in middle school that the South seceded because of states rights, and the argument "They only left because racism" was unfair, if not blatantly false.
However, if you look at the Ordinances of Secession for most southern states, slavery is right there in the paragraphs. So slavery was obviously on their minds. If you look at Arkansas' Ordinance of Secession though, (http://gen.1starnet.com/civilwar/arord.htm) it puts the total blame of war on Lincoln's call for troops against the rebels in Fort Sumter. If you look at the Wiki page on Arkansas, we even refused the initial call for secession until Lincoln openly showed that he would go to war to preserve the Union.
I checked the Wiki here, and all questions about slavery and the Civil War include answers that either don't specifically speak on Arkansas or make assumptions about the South that lumps Arkansas in with everyone else. Arkansas, contrary to popular belief, has done some great stuff in its history. And I refuse to believe that we left the Union just because Louisiana liked slaves. Unless, of course, there are some PhDs that know that to be the case.
So, how much did slavery factor into the Arkansas secession? Slavery wasn't widespread in Arkansas, and while we were undoubtedly racist, we seemed more worried about doing what we wanted to do than actual, full-blown slavery. Is Arkansas actually one of those states that seceded over states rights, or were we racists that hid our hatred in protecting our fellow southerners?
Here's what Daniel Sutherland, professor of history at the University of Arkansas, wrote in "A Savage Conflict" (Chapel Hill, 2009):
"Arkansas faced perhaps the most unpredictable situation [during secession]. The state was betwixt and between, not part of the Deep South but not currently threatened by border war. Yet, Arkansas could but think of Missouri as an extension of their own state, which meant that if their northern neighbor fell to Union rule, the war must inevitably roll southward over them."
Hope this helps! The book is pretty good.