Since the questions have no accepted answer, they fall within the set of questions to be bumped. Even if an item is tagged completed, if no answer had been accepted, it is considered an active and open question. This happens a lot when users do not come back to follow up, or do not agree that it is completed to their satisfaction.
We are discussing with Jeff reducing the questions bumped on Meta as a whole, since the site does not generate a lot of traffic.
A more detailed explanation of how it works can be found herehere