Could not the complaints have been cumulative, festering over the time of many years, and fresh in the memories of many, adding to the overall mood of the South? "A long train of abuses". By 1860 couldn't the South have just been finally fed up?
During the period between 1850 and 1860 tariffs had been reduced considerably. The Morill Tariff was defeated in the Senate in 1860. So if tariffs were indeed the cause of secession, and there is nothing that suggests they were, then the timing is suspicious. Why secede over something that had been going down for years and which did not impact you very much to begin with?
"[Mr. Stephens} The next evil that my friend complained of, was the Tariff. Well, let us look at that for a moment. About the time I commenced noticing public matters, this question was agitating the country almost as fearfully as the Slave question now is. In 1832, when I was in college, South Carolina was ready to nullify or secede from the Union on this account. And what have we seen? The tariff no longer distracts the public councils. Reason has triumphed. The present tariff was voted for by Massachusetts and South Carolina. The lion and the lamb lay down together-- every man in the Senate and House from Massachusetts and South Carolina, I think, voted for it, as did my honorable friend himself. And if it be true, to use the figure of speech of my honorable friend, that every man in the North, that works in iron and brass and wood, has his muscle strengthened by the protection of the government, that stimulant was given by his vote, and I believe every other Southern man. So we ought not to complain of that.
[Mr. Toombs: That tariff lessened the duties.]
[Mr. Stephens:] Yes, and Massachusetts, with unanimity, voted with the South to lessen them, and they were made just as low as Southern men asked them to be, and those are the rates they are now at. If reason and argument, with experience, produced such changes in the sentiments of Massachusetts from 1832 to 1857, on the subject of the tariff, may not like changes be effected there by the same means, reason and argument, and appeals to patriotism on the present vexed question?"