I believe this is probably happening whenever a new change has been made to boards.
For example, it may have just occurred recently because category emblems were just added. Basically it's the boards detecting a new change and attempting to auto-update.
As long as changes aren't being made, it shouldn't happen. When it does happen, its indicating something was just changed and that a refresh might be required.
This is just my current theory though, I'll keep in eye on it regardless.
How frequent is this?