First of all, the document was never published and is not authenticated -- could easily be some later construction. Indeed, it sounds like a later construction, compared to the other secession documents.
But even so, read it again, pal.
It contains not one word about some major "usurpation" or "abuse," "injury" or "oppression" which did happen, but only expressions of fear about what might happen in the future.
Southerners had no real reasons to secede, nor to seize Federal properties, nor to fire on Federal forces, nor to declare war on the United States.
But they did it anyway.
The South chose its fate, and should now be happy with it.