re: How would a major secession event play out today?

There was no FBI and not much of a standing army in 1860. So there was limited federal forces in place to confront a nascent rebellion for Buchanan and Lincoln.

At some point, a secession movement would have to move away from an online movement to a very public presence and show itself. I'm having trouble imagining what kind of issue would attract sufficient critical mass to exist in public.

Now if say, ISIS or secessionists, detonated a nuclear bomb in D.C. and wiped out the federal government, then who knows. This could result in a de facto secession by default. Not exactly what you probably had in mind, but that would result in state and local governments having to assert more authority at least for a while.

Let's all hope this sort of thing doesn't happen. But that's about the only situation I see resulting in de facto secession.