The revolution takes place in the hearts and minds of the population before the war. The war happens after the revolution and the secession.
The founders and the colonies seceded from the crown after the revolution of heart and mind turned the population against their former loyalties. The war that followed was entirely the doing of the British in their attempt to keep what they judged was theirs.
Secession happened because a significant percentage of the population had a change of heart and loyalties that were brought about by abuses and the people getting a taste of what real freedom was like.
Secession... They no longer wanted their lives and future to be influenced by those they had been previously associated with, so they proclaimed themselves sovereign nations and then created a loose federation of those nations and called it the united states.
It was a secession because they did not sail over to England and sack the king and the parliament, and whomever else they could get their hands on. They simply stated their independence from those that had previously controlled them then sought about governing themselves.
Secession is the moral way to end an association with folks you would be just as happy to bury.
A legal secession may be the best way to end an association but the Confederate democrats did not care about legality but were instead tyrants who usurped such power and were ruling tyrannically over citizens of the United States.