You can argue that it was set off when the Northern states passed laws that freed any slaves who set foot there and the Southern states demanded that the laws be abolished and all states be required to enforce slavery. They didn't get their way so they decided to throw a tantrum.

Of course even with that slavery remains the lynchpin of the whole thing.

Because the Confederacy was against states' deciding things for themselves. They wanted to force other states to comply with their demands.