This touches on both How can moderators prevent an 'in-crowd' forming? and How do you deal with a back-seat moderator? issues, but I don't think it's the same angle.
I have noticed, in myself but also in others, that some users can follow a specific path through a Stack Exchange community. They come in, maybe with a question, but quickly discover that the community appreciates their answers. They adapt well to the system, and rapidly gain reputation. Attracted by the reputation game, they develop intrinsic motivation of returning to the site and participating, and become a valued community member. Their reputation gives them access to a wide range of community instruments, and they are present enough to take a look at every single question in a small community, or maybe every question in their tags in a large community.
At the beginning, everything about the site is interesting. It is fun both to read and answer questions, and sometimes to ask one's own. But at some point, there is some habituation. And the user starts disliking questions which fail his personal expectations. For example, he may start feeling like a real expert (because he notices he can answer 85% of the questions on the site really well), and when he is confronted with a question outside of his area of expertise, he starts arguing that this kind of question should be off topic, because it really isn't about the main point of the site. Or he just gets bored of the newbs who find endless ways of running headfirst into the same wall and ask what to do. On Stack Exchange Cooking, it is interesting to explain once in depth how the protein in muscle behave under heat and when they are overcooked and your meat becomes a shoesole because of this. But there are at least two questions per week whose short answer would be "just stop overcooking your food", mostly posted by new users. The bored user can start downvoting questions just because he is bored by them.
Also, when new users come in who unwittingly break a rule, they can be met by this kind of user who brisquely tells them off. The old user doesn't want to be mean, but in his impatience he doesn't realize that the new ones couldn't know the rule and need more guidance than just seeing a close vote on their question.
A handful of such users can form an in-crowd which is quite hostile to new users, and is one of the reasons for the problem mentioned in the first linked question. The high presence and little patience leads to the user frequently taking actions which are correct community moderation (so they can't be told off for overstepping their rights), but done in an unfriendly way towards hapless newbs. This is disruptive to the site and its openness. Also, I see it as a bad sign when a single person starts seeing a community site as his personal playground (and I confess I have been guilty of this to some extent).
How can we as moderators support these users without alienating them? How do we stop them from influencing the site too much and driving it into a direction the middle-tier rep users disagree with? From scaring off new users who feel stupid after these users' intervention (because they dared to ask a question which was too simple and/or boring and got a searing comment, or because their question got downvoted or closed without explanation)? How do we help them divert the energy they are investing in the site in more community-oriented behavior?