Actually, they had been fluctuating between 1816 and 1860. The real spike in the tariffs came in 1828 with the tariff of abominations which actually caused a rift between Jackson and Calhoun because of just how targeted the tariff was toward the south. In effect, it made export of cotton to Brittain so costly that the south had only one option: sell their resources at a markedly reduced rate to the north who then manufactured goods and sold them at exorbitant prices to the south. It was a truly abominable act on the part of congress to pass that tariff and that is where the rift between the north and the south pretty much became irreparable without bloodshed.
That tariff led to a series of heated arguments between the north and the south which culminated in the Nullification Crisis...pretty much a situation where the Southern states convened popular conventions and drafted their own legal philosophies regarding the Constitution as a contract between states and not a supreme law of the land, giving South Carolina, in particular, the right to nullify the tariff (which by this point had been redrafted as the Tariff of Abominations of 1832, reduced by 10%, leaving it still at a staggering 35%). In response, Jackson had Congress draw up the Force Bill which granted the PotUS the power to call up state militias and use the army and navy to put down insurrection. At this point, Calhoun left his post as VP and ran for senate. The conventions reconvened and passed rulings that nullified the Force Act. This is basically the stage that was set for Lincoln upon his inauguration. If he had really wanted to keep the peace and unity of the nation, he would have done something about the tariffs, but instead, he reaffirmed the tariffs, pleasing his northern industrial supporters but alienating the agrarian south. In fact, when South Carolina did secede, he was quoted as having said "But what will become of MY tariff"? No, Lincoln was not interested in peace and unity at all, but rather keeping his industrial northern supporters happy. Even in the 1800's, politicians were whores to their business interests.
The issue of slavery didn't even come into the picture in force until British Abolitionists, bereft of any reason to support the south now that the Tariff made trade with the south unprofitable, began to kick up a storm about slavery. The pressure to end slavery in the south came from abroad more than anywhere else and it was used to leverage other diplomatic issues, which Lincoln ultimately capitulated to because he couldn't handle a civil war at home without giving it some meaning. The slavery issue was the perfect excuse for prosecuting a war to keep the union intact but he didn't go freeing slaves out of his humanitarian interest in the plight of the displaced African at all.