The problem was unilateral secession. By that, I mean the attempt by one state to fundamentally alter the Constitution and the Union by itself, with no input or coordination from other states. Such action went against the spirit of the Constitution, which regulated the relationship among the states.

Secession did not simply affect the individual state which seceded, and thus unilateral secession was really an arrogant and aggressive move.

Secondly, the timing in which the Deep South tried to carry out secession was questionable. Lincoln's election, though distasteful, was legal. He had not come into office, nor had any specific Southern rights been denied, during the first spat of secession ordinances. The Deep South tried to carry out what may be called a preemptive strike by seceding BEFORE there was any actual attack upon them. Southern Unionists, such as Alexander Stephens, criticized this excessive action during the secession debates.

The timing also challenged (or made a mockery of) the American democratic system. It tried to set the precedent that a state (or body of people) could simply reject a legal election or political decision by quitting. In other words, a minority didn't have to submit to the majority decision...they could just run away. That was a break down of order and the Constitutional system.