The more I think about it. The more I remember reading about Robert E. Lee being at Harpers Ferry, the more I do wonder if that event was more powerful in his decision than just that Virginia had left the Union. Lee had seen war before. Lee had seen it as a cause with a real reason and followed orders. If he saw, or believed this type of activity such as Browns was not going to stop, he pretty much saw the writing on the wall. It's quite possible this was a wake up call for others also. Adding Browns fervor, the other events taking place and the underground Railroad, it seems there's only one way for any of them to go.
I'm not trying to get off topic here but have been thinking the one example I can understand on the problem of States Rights being over turned, ignored or just plain attacked when the state was doing what it was designed to do---protect the life and welfare of its people. I would think anyone would be hard pressed to believe Arizona was not exercising its right to do what it had to. The real kicker was that it was condemned for this by the Federal government who should have been helpful but instead were just AWOL. That in itself is enough to make a state want to reconsider and reread the fine print in the States Rights clause. A different issue in 1860, but being ignored on the federal level had to have tossed some fuel on the fire. It's just human nature.
There are so many aspects to this story and just a bit from each is enough to explain just how concerned the Southern states were and why they took that drastic step.