I have seen this in many communities, and my opinion is that it occurs from large growth and no monitoring trends in attrition with a coincidental "that's how the community voted; too bad, so sad" mentality that, unfortunately, generates a shout-down chamber, even if only on a limited set of topics.

This occurs, right now, here on HN, and IMHO it comes from the comments feed, a feature I find only useful to moderators and manipulators. Even the mention of certain topics from even a meta-discussion perspective receives down-votes, sometimes within seconds. This seems further amplified in communities where senior community members are granted super powers.