The south was about to lose the power to preserve slavery through their partial control of the federal government. When Lincoln was elected they lost that power and the next few states would most likely have been free states which would have given the free/anti-slavery states control of the Senate, Presidency and the House (I think).
One of our nation's founding principles was that a government only retained legitimacy so long as it enjoyed the consent of the governed. The South withdrew that consent, thus delegitimizing the Union as to the Southern States, after which point, the Union became tyrannical by definition.
