These are the people with an unshakable belief in their own superiority, at least in some field. It's not that they feel a need to demonstrate superiority over others through condescending feedback. It's that their superiority is implicit in their world-view. They don't necessarily even realize they're being condescending. It's just the natural order of things.
I can say this with full confidence because I've been that person. As a teenager, I was insufferable. Not only was I a freakishly good at bowling at 15 (making hundreds of dollars per weekend from "pot games" and with multiple 300 games in a single league), but two years earlier, I had also become the youngest person in my state to enroll in college classes. I tried to be nice to people and everything, but I just assumed I had more intelligence, dexterity, constitution and possibly other D&D stats than they did. If people didn't like my attitude, sometimes I actually felt sorry for them, assuming that jealousy was the reason.
Now in my 30's after not having done anything amazing since growing up, I've become less confident and also less condescending. Rapidly picking up coordination based sports isn't really that important in daily life as it was as a teenager. And I certainly don't assume or even believe that I'm any smarter than other people. On the contrary, I'd say I've become tangibly dumber. I might still be condescending from time to time in my weaker moments, but nothing even remotely like what I did unconsciously half a life time ago.
I think the sad fact of the matter is that many geeks just have poor social skills. While some are content to just be quiet, others feel the need to overcompensate. Luckily, I haven't met too many of the latter in real life, but you see it everywhere on the Internet. The dehumanizing nature of interacting with a computer screen seems to absolve people of their need to act socially, so they act like fuckwits. On 4chan it takes the form of blatant aggression, although anyone familiar with the place knows it's all part of the joke. On reddit people make snarky replies that make up for their lack of content with the sharpness of their barbs. Here on HN it's more muted. While no one is a blatant asshole, there's a lot more "downvote and ignore" that goes on. This contributes hugely to the hivemind effect, because there aren't arguments given as to why someone is wrong, you just have comments that don't jive with the rest of the community getting hidden. This, in turn, causes people to self-moderate, so that they only post opinions they think the rest of the community will agree with. Obviously these are oversimplifications, and the issues I mention aren't particularly common, they're just how the same issue manifests itself in the different communities.
I suppose the insecurity doesn't have to be about domain knowledge, either - it could be about communication skills. Perhaps some people giving condescending feedback don't feel like they are able to communicate in any other way. They experience the desire to contribute, but aren't confident in their tutoring skills and don't want to risk sounding stupid by bungling an explanation.
This is admittedly pure speculation...
I think the bit about superiority may be true: it is a way to demonstrate superiority, but I don't believe that immediately indicates low confidence.