The only usurping that the Southern states were concerned with was the fear that Lincoln would prohibit slavery in the territories. Doing away with slavery would have eliminated that problem so it's back to the original question - had the Southern states done away with slavery then what was there to secede over?
Again you are judging by today's standards, Morrill Tariff was enough of a catalyst back then. Back then people took their states rights seriously.
To show how far we have strayed, how bass ackward everything is take the following test.
Most people couldn't answer the most basic questions about their state legislatures without looking it up on google. Can you answer these simple questions regarding your state legislature(without the aid of a search engine)?
No, you seek only to distract with this question. The real question is, why in the world would anyone think it is fine and dandy to strip the states of their basic sovereignty?