Almost all items in DSpace have a dc.date.accessioned because it is automatically assigned when the item is created. There are two conditions which can occur that causes an item to get lose it's accessioned date.
1) A person with editing privileges on an item removes the dc.date.accessioned metadata field.
2) The item's accessioned date is created via the InstallItem.restore() method.
When one of these conditions occur the item gets inserted into the browsing sort table with a null value. When ever a recent submission query is run these items will always be returned first because Postgres and Oracle always return NULLS first when executing an ORDER BY clause. This means that these items are permanently penned at the top of the RSS feed and recent submissions sections sections.