I love/hate to watch the same thing happen (on a small scale) to subs on reddit. Some communities can grow and grow while maintaining a culture, but most have some drift, or even a complete change in the userbase.
/r/minimalism is a good example of this. Not to sound pretentious, but I was one of the first hundred or so subscribers to it. It's been bad for a few years now but in general it was killed by a massive influx of users who all had their own definition of minimalism, and they came before the sub had enough time to have its own culture/definition.