The ordinances of secession were written by the secessionists themselves at the time. No victor had anything to do with them.
If you don’t believe that the south rebelled because of slavery you are gravely mistaken. I would recommend educating yourself by reading the ordinances of secession and the written accounts made by the secessionists at the secession conventions.
Silly on the face of it. Why didn't the five Union slave states secede? They had slavery too. If "slavery" was the reason states seceded, then why didn't the other slave states also secede?
In fact, Virginia lists as it's primary reason for seceding the abuse of power from Lincoln in calling up an army to deny freedom to other people who wanted out of his control.