Today I noticed this on a post:
What exactly does this mean and what triggered it?
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The community user will randomly bump old questions to the front page in order to attract more attention. It will only bump questions that meet certain criteria:
This action will take place once an hour. If no questions are found matching these critiera, the bot does nothing.
Recently, a change was added to make the bump more obvious. This is the banner you saw in the post (also known as an annotation), as well as a note added to the Timeline and revision history.
Rest assured that while Community is a bot, it will not rise up to become Skynet (yet, though the SE devs have said otherwise). It's just a helpful little bot to give old questions a chance to see the light of day again and be dealt with accordingly.