It's the threads that are popular right now, it has nothing to do with when they were created or how many views, replies, etc. From other thread:
Indeed! lol... in case you (or anyone else) is wondering how we are determining what's being "viewed now", internally it's via the Google Analytics Realtime API: https://developers.google.com/analytics/devguides/reporting/realtime/v3/reference/data/realtime/get The API usage and widget are generated by our "Better Analytics" addon for XenForo: https://marketplace.digitalpoint.com/better-analytics.1787/item The WordPress version has the same sort of option (ability to add a widget based on Realtime Analytics API that shows popular posts being read right now): https://wordpress.org/plugins/better-analytics/ Way more info than you wanted I'm sure. lol
Are they made popular by default? For instance, this one got it last reply in 2012, but it's in the "popular right now": https://forums.digitalpoint.com/threads/new-t-shirt-dropshipper-for-anyone-interested.2100395/page-2 I keep ignoring them, I wonder how many forum users do that too.
Nope, there is no defaults... If no threads are being viewed right now, nothing would show. That particular thread has more than 33,000 views... probably people viewing it coming in via Google search or something. {shrug}