They could have paid the highest fares on the plane for the seats they got bumped out of, so would be the obvious choices if the alternatives are unsuitable for or unwilling to upgrade.
Also, a Gold card holder or other high status member who would normally get priority when looking for upgrades may have been traveling with kids who they could not be separated from. Usually children don't get free upgrades (in my experience anyway) as the premium cabins are better suited to adults (of course children can be booked into those cabins on almost all airlines).