I'm new to this forum and realize this content was published long ago (2015) but I'd like to thank you for your candor on this matter. I found this post when doing a search on "subtle rudeness".
I find many people constantly use subtle rudeness and my conclusion is that they think they are particularly clever in being able to downgrade/dismiss others in public in subtle ways.
Maybe its just my perception. I grew up in a house with two sisters and a mother. My father was in Europe working most of my adolescent years and my mother and sisters reveled (and still do) with subtle , and not so subtle, downgrades.
Without going into details on the numerous occurrences that I can remember in my family, I am already sensitive and been trained over a lifetime to look for sideways comments. I notice it perhaps much more than some people. Its hard to say because the art of their science is to sow doubt surreptitiously so they can never be questioned with the content. Their method is to throw a non-addressable conversational sucker punch to which you cannot reply.
When I was young it started with passive-aggressive teasing. When I got older and addressed that 'teasing' is not a fun way to express yourself it only exacerbated the problem because then the 'teasing' went underground into public surreptitious rudeness.
Don't get me wrong, there are plenty of people who exist peacefully with each other without trying to build themselves up by downgrading others. Usually, my attempt in my personal life is to distance myself from the people who can't build their own self worth without degrading others. This can be difficult because I don't usually find out that the person is that way until becoming socially intimate with them over a period of time because many do not show this face in public. The ones who do are easy to weed out early to avoid future issues with that behavior.
I guess I cannot confirm whether this is normal behavior. When I have attempted to discuss this with many they act as though I am overly sensitive (both in and out of a family setting). I usually casually blow it off when it comes up occasionally as an issue. It is somewhat human nature to compliment yourself relative to another. But when it comes out as downgrading others constantly (usually accompanying gossip, where you can not only downgrade a factual personal attribute, you can extrapolate from particles of truthful attributes and create a seeming justified alternative narrative that can resonate as a story) I really prefer to minimize those relationships when possible.