The literal truth of it is the South was producing a sh*tload of money that had been going through New York with Washington taking it's cut, and that was suddenly about to be cut off.
You have to be a fool to think New York and Washington DC (by the way, the same two cities that mostly run our lives today) wouldn't move heaven and earth to prevent the loss of 200 million dollars per year in 1860 dollars.
Isnt it usually about now when you break out that awesome graph?