It looks like y’all may be talking past one another.
Most pro-unionists claim that the cause of the Civil War was slavery because the rebels themselves said that it was - in their rhetoric, in their declarations, and in their actions. It wasn’t that anyone (abolitionists or otherwise) was ordering the south to give up their slaves - in this very thread we’ve seen plenty of evidence that this wasn’t taking place - but that the fire-eaters of the south had seized upon the pretext of Lincoln’s election to execute their plan to disband the United States.
Of course they say other things, but those are all rationalizations meant to excuse their behavior or salve their consciences.
Ultimately I agree with you - no rational reason existed for the South to secede.
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.
Ultimately I agree with you - no rational reason existed for the South to secede.
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Virginia seceded AFTER Washington informed them of the number of troops demanded to be supplied to the Union by the State of Virginia. At that time, Robert E. Lee resigned his commission in the United States Army and fought for his native Virginia.
The taxation of the south was reason enough to secede and forcing the south to supply the money for the union to operate was Lincolns reason for the war. He moaned and groaned how was he going to run the government without the southern taxes.