Based off of my personal experience, my theory is that this kind of tonal disdain stems from racist people who get frequently called out for their racism, only they don't think that what they say is racist. So instead of understanding it, they internalize it as "everything I say is racist!" And so by feeling unfairly judged, even when discussing the simplest and vaguest references to race, they self-police their own words but feel disgusted that they're forced to do it, so it bubbles up and seeps through their tonal delivery.
That's an interesting explanation, well said!