Rationality means you accept prevailing conditions and try to work within them.
Sometimes people come to see the current condition as so unpleasant or the possibility of a radical break with it as so promising that they decide to take the gamble and spin the dice. It's irrational, sure, but at the time people think the break is so bold that it just has to work.
Northern fear about the spread of slavery west was rational. There was no reason slaves couldn't have been used in mining or ranching, and look at where we actually do grow cotton today Southern fear about the threat to slavery was likewise rational. As new free states entered the union it would be harder to keep the country half slave.
It's been said that slavery was more secure within the Union than outside of it, so secession was irrational. There's a lot of truth in that. But I suspect slave owners were so mesmerized by possibility that their world was eventually coming to an end that they decided to risk the gamble of independence.
Something similar happens often enough in history. A country sees itself in a position of power that it knows won't last, so it decides to take advantage of its momentary power to get rid of its enemies while it can.
Considering the previous 40 years of contention before the civil war, the argument was not so much about slavery itself, but the expansion of slavery. The money to be made from slavery by 1860 was only partly from the labor they could provide. It was also from the slaves themselves. They became very valuable, and they reproduced at a much faster rate than the white population.
To maintain that value for a rapidly growing population of slaves required opening new markets to sell "excess" inventory. Without new markets, the slave states would have in a generation or so been drowning in excess slaves. Their market value would have collapsed, fortunes would be lost, and the odds of slave uprisings would increase.
Lincoln, and the Free Soil Republicans, whose only promise was to stop the expansion of slavery to the territories, was indeed a dagger to the heart of the slave powers.