Before we start analyzing a bundle of switches, it would be helpful to make sure we're all on the same page as to what, exactly, a bundle of switches is. If you're thinking a whole bunch of electrical devices that turn lights on and off, guess again. See, before Thomas Edison changed our lives and our late night reading marathons with the light bulb, using a switch (as a noun) to switch (as a verb) someone meant that you were giving them a good ol' fashion whipping (or switching, as the case may be). Classy, right?
So now that we're all caught up, let's get into our text. On the night the Creightons are threatened, it's with "a bundle of switches tied together with a cord" (6.133). This bouquet isn't intended for a public flogging though, and instead it's a sign that something worse is yet to come. Oh goodie—something to look forward to. That special extra something turns out to be Matt's barn being set ablaze in the middle of the night. Literal burn.
You may wonder why all the passive aggressiveness. Wouldn't is be easier to just confront the person you have beef with and get everything out in the open instead of doing a midnight run with symbolic threats? Probably, but intimidation is part of the bad guy game, and a key component to harassing the Creightons is making them have to constantly look over their shoulders for the next shoe to drop. It's pretty cruel, but also pretty effective.
And of course the bundle of sticks only appears because some folks aren't too pleased with Bill's deflection to the Confederate army. Because of this, they remind us that the Creightons aren't just a family divided by the war, but also part of a local community that is divided, and a nation on the brink of splitting in two. That threatening the Creightons over Bill's decision harms innocent people foregrounds the book's anti-war—and anti-violence—message.