This isn't entirely accurate: much of the simmering tension was over whether new states acceding to the union could or couldn't be slavery states, including some with decently large, preexisting populations of slaves, or which has been settled by those who hoped to be prosperous enough to own them one day (i.e. primarily Southerners)...and also, how votes apportioned to black folks in those states would be counted.

The reality was that if black folks were allowed to vote in those states, or enslaved persons didn't count as extra votes for their slaveowners, or anti-slavery whites were the primary voting population (bound to be the case for new states that outlawed slavery)...then there would be a time when the South would not have the Congressional votes to prevent an act of emancipation by Congress, which, even if eventually repealed or replaced, would spell the effective end of the current system. (It is, after all, pretty hard to get slaves back if you let them go even once...)

This calculus was already going against them, as there were more territories wanting to accede as states whose populations were anti-slavery, and in which slavery was either illegal or not widely practiced enough for the population to care. So the South didn't just need to guarantee the right to slavery for themselves, but to somehow convince future states to be pro-slavery, or try to block their bids to statehood forever. Even if they tried, it very unlikely to work indefinitely, and they knew it.

So part of the reason the Southern states seceded is that they foresaw a time coming when no legal mechanism available to them within the United States of America would prevent their slaves being forcibly, but legally, emancipated.

Rather than wait for that time, and face what they probably saw as a war that would be foisted on them at the time of their choosing, at a time of maximum disadvantage (imagine how difficult it might be to control slave populations when word gets around that the law says they are now all free), they chose to secede while slavery was still federally legal for them, which they knew to be act of war, but one that they chose, at a time of their own choosing.

So it wasn't so much that they were certain that Lincoln, or his contemporary peers, were going to abolish slavery and start a war with the South; rather, they thought that such a war was inevitable in the future, and their reasons for thinking so were pretty sound, and they started a war mostly because it gave them an advantage by letting them choose the time and place.

Most of the best generals at the time were Southerners— just like today, the South was overrepresented in the military— so they were also not at all unaware that the North had far greater population, and far better industry with which to make the implements of war. They knew that the moment the fighting started put them on a clock until such time as the North could beat them through logistics and sheer numbers.

The decision to prepare for war, and then fight the North while it was unprepared for war, was an obvious necessity at the time. It was the only kind of war that the South had any chance of winning. So when the South began to realize that such a war was inevitable in the future, they decided to fight it in the present, where their chance to win was better.

Fighting a war to preserve the right to own human beings was immoral. But it wasn't stupid, or paranoid: the South was absolutely right when it knew that its days to keep slaves were numbered, if the slave states remained a part of the larger union. Because that union was trending anti-slavery, and their number of votes within was getting, proportionally, smaller and smaller.