And how in the world can the words that I said
Send somebody so over the edge
That they’d write me a letter sayin’ that I better
Shut up and sing or my life will be over
Here's how: You tell someone something that challenges their conception of self. Music, sports, and religion are a few of the existential things we use as signs to mark out our own identity, as well as signal that identity to others. Largely because existential objects are hard to display we create concrete ones to denote particular aspects of ourselves and display them to the world.
When you challenge one of these identity symbols someone is displaying, or force them to rethink (read: reject) it they will reflexively attack. You’re seen as a threat to their core being and in today’s America no one has the right to question your identity, much less make you question your own.
So, you like country music, America, and the Dixie Chicks? Then you probably identify with their “ethos” (attractive women, low taxes, trucks, Republicans, guns, beer, Team America, etc). Then they go and force you to question their (read: your) patriotism... Well god damn right you're going to get even for this.
It’s the survival instinct turned existential. You feel as though you're under attack, except nothing real was being threatened and your unfamiliarity with discomfort has caused you to retaliate in the Real World™, not just the one in your head (they’re the same to you but no one else cares; you aren’t the star in other peoples' movies).
Why do people react this way? Short answer: branding. Long answer: psychology plus branding. Next time you are reading through a National Geographic, take a look at the clothes the people are wearing. What brand are they? Wait, what? They don't brand their clothes? What the fuck kind of backwoods... Okay, never mind, the question isn't why they don't; the question is why we do.
Everybody has to feel superior to somebody, but it's customary to present a little proof before you take the privilege - Truman Capote
Quick history lesson: Prior to WWII America was mired in a depression and life was generally Hobbesian so Capote's proof was in ample supply. The people at the top had good food, the people in the middle had some food, and the people at the bottom had boot straps. Then we killed a bunch of Nazis and Russians and suddenly everything went all topsy-turvy. America became Great (capital G) and The Dream (™ Levittown) was within everyone's reach.
Trouble was, Capote kept getting confused because everyone now possessed the old status symbols. Not to worry, capitalism can fix this. Back in the days of the first and second Reichs, branding didn't really exist if you weren't a cowboy; there was merely advertising. (Etymology: brand from old Teutonic brando: to burn; advertise from middle French advertiss: to take notice of). During the post-War boom companies began to see advertising as a means of showing the public a corporate image, instead of simply showing off products. As the decades moved on (watch Mad Men if you don't know what's coming), advertisers shifted their message away from the product and onto the brand. Moreover, they became the arbiters of each brand's place in the zeitgeist since no one else stepped in quickly enough (oh, hello FTC/FCC).
The critical distinction here is that a brand carries with it existential baggage that a product does not. The explicit goal of advertisers is to make you identify with the brand and then purchase the product. This is why Apple's ads were, "I'm a Mac" not "Hipsters in coffee shops are Macs."
A self-idea of this sort seems to have three principal elements: the imagination of our appearance to the other person; the imagination of his judgment of that appearance, and some sort of self-feeling, such as pride or mortification. - Charles Cooley
Cooley is saying that two thirds of your view of yourself is imagined. (Etymological note: imagine's root is image which shares its own root with imitate). Which brings us to the crux: What determines your imagination of someone else's conception of you to be? (Whew, that was a lot of meta-bullshit, but bear with me).
A quick gedanken: What is the function of a chair? Whatever your answer, it's wrong. Without the context it's impossible to pinpoint the function of a chair. A chair on a table under a lightbulb does not have the same function as a one at a desk with a typewriter, or as one on the beach in Mexico, and so on. This is a simplistic but the point ought to be clear: one cannot even ascribe purpose to a concrete object without an appreciation for its context. It's likewise impossible to ascribe judgements to other people without appreciating our shared context.
In this case the Dixie Chicks represent a conspicuous method of identifying with an ethos. The problem arises when they do things that you oppose. You've already internalized a static construction of the Dixie Chicks (and the attendant ethos) and then promptly forgotten that that construct is actually just three chicks from Dixie (ostensibly). This is because cultural constructs like bands (or religions) are defined far more by the amalgamated cultural identities of their followers than they are by the constructs themselves. This is how bands come to symbolize generations or social movements even if they never participated in them. Unfortunately (for you) no part of this process strips the actual people of their autonomy (the record executives usually do that, but they whiffed on this one). This is why Maines was able to stand up and say that she was didn't want the war and that she was ashamed the President was from Texas (seriously, that's all she said that got people so pissed off...).
What happens now is that you need to find a way to assuage the deluge of dissonance. The most typical method of doing this is to decide that the emblem no longer represents the things you were trying to display (so now the Dixie Chicks aren't patriots and may be traitors) except that you have invested time and money (same thing, really) in your identification with them. This (instantaneous) realization leads to rage because you defined who and what the Dixie Chicks stood for then they took your investment of time and money and, worst of all, tricked you.
So you lash out.