Who is the Community user?
I found this question shown on the "active" list, as in the following picture:
But I went though carefully on this page, none of the operations was done by "Community".
Is this a bug?
No, this is not a bug.
The Community user automatically bumps "old", unanswered questions to get them some attention.
This is confirmed by the "about me" section in the Community user's profile: