Hello, x, good to see you again.
You make a good point, and the question then is to distinguishing root causes from proximate causes or triggers. I was merely riposting that Lincoln's election was the triggering event.
It would be interesting to speculate on the likely course of secessionism had Douglas managed to hold the National Democracy together to turn back Lincoln's challenge for national leadership. Keep in mind that Canadian secessionism has now been held at bay for 40 years, or about as long as Clay and others kept it at bay in the U.S., and the Quebec separatists now seem no stronger, and indeed weaker in some ways, than they have been in the past.
I'm not offering Canada as a perfect proxy, merely making a point about this "historical inevitability" that people talk about, who've been catechized by followers of Toynbee and Marx.