One of the duties of the community user is to
Randomly poke old unanswered questions every hour so they get some attention
according to the Community user FAQ.
This seems to only happen for questions with non-upvoted answers, but not for questions without any answers at all. I did go through a large number of posts on two sites where I am a mod (so I can see deleted answers), and I found no examples where the community user bumped a post that never had any answers at all. I always thought that the community user was supposed to also bump entirely unanswered questions.
Is this a bug, or is this intentional and the description of the community user is simply misleading?