I believe the Community User "bumps" older posts that do not have an accepted answers and I believe this changes the latest activity date without displaying any viewable changes.

From the Community User's profile:

I do things like

• Randomly poke old unanswered questions every hour so they get some attention

I believe the Community User "bumps" older posts that do not have an accepted answers and I believe this changes the latest activity date without displaying any viewable changes.

From the Community User's profile:

I do things like

• Randomly poke old unanswered questions every hour so they get some attention