[jeffersondem]

“Was it over “money flows from Europe” or “Northeastern power brokers” or Northerners “raping” the South economically or “confiscatory tariffs”? Naw, it was none of that because not one Southerner in a thousand understood or cared about such things.”

I have heard a lot of charges made against Southerners - one expert wrote that James Madison was a yapping dog, for example - but I don't remember it ever being said that southerners didn't care about excessive taxation.

[BroJoeK]

jeffersondem: "I don't remember it ever being said that southerners didn't care about excessive taxation."

"Excessive taxation" in the eyes of beholders.
Southern Democrats worked for lower tariffs and by 1860 they were as low as ever, less than half of Andrew Jackson era "tariff of abominations".
Republicans wanted higher tariffs to protect US industries, but so long as Democrats remained united and strong they could block or minimize increases.

So the new Morrill tariff never passed until after Southern Democrats walked out of Congress.

And why did Southerners threaten in 1856 and then secede in 1861?
Was it over tariffs or any other similar matter?
No, it was over Republican threats to slavery -- that's what they said at the time.