On the group overview page at http://yournetwork/groups , the default sorting order is "most active first", and I have no idea what algorithm determines this ranking. However, I am pretty sure that 'frequency of last visits' or something similar plays its role here: people visit a group, don't do anything, but this is still counted as activity.
Result: a self-perpetuating ranking system. New users land on the group overview page, see some so-called 'most active' groups, click them, thereby visiting them, thereby increasing the activity level of such groups ---even if no activity ever takes place there.
It would be better of the default sorting of the group overview page would be "last activity first". Even "random" would be good. Or even better, let NCs decide upon the default sorting order.