The pages panel will expand all the pages you have clicked on the revision number (circled below) for the current session (i.e. until the tab is refreshed). It happens when you're trying to expand only one page to view/adjust overlays and it'll sometimes expand the pages when starting a new measurement with a takeoff.

I think it maybe only happens when you have multiple pages with overlays?
Also, side note- can we filter out takeoffs from the pages panel view? I would like to be able to just see the pages and the revisions sometimes when overlaying pages. Scrolling through all the takeoffs to realign the page multiple times is a bit of a workflow annoyance.