I have noticed recently that in certain communities, anywhere from 10-50% of the questions are about the same thing. (having the same tags) When this happens, should I attempt creating a new community for this topic, or just leave it alone. It can be annoying when I know nothing about that particular thing, and most of the questions are about it, making me have to sort through more questions to find one that I can answer properly.
We don't generally split off subjects simply to give them their own space (sometimes referred to as "vanity sites"). In our site-creation process (Area 51), folks often try to employ this logic — "There are so many questions about [Java] on [Stack Overflow], we really need a new site."
That makes little sense.
When a subject becomes really well-supported on a site, the last thing you want to do is send half the community off to another site to start over again from square one. For one thing, you lose all that momentum from folks who have become accustomed to answering on a successful site. And by splitting that tag off to another site, you end up with a terribly disjointed legacy for everyone left behind.
At what point does it become necessary to create a new community for a topic?
Who knows? There isn't a right answer here.
What we do have, though, is a process for suggesting new communities.
If a suggestion gets enough of a following, we here at Stack Exchange will create a site for it to try it out and see how it goes. If the trial period goes well, it will become a new community site.