Perhaps staff brought them while he was still in office. Perhaps movers just grabbed whatever from the white house when he moved out. Perhaps the items in question were planted/mixed in with their belongings so it could be used against him later. The truth is, we don't know why they were there, just that they were there... at least according to that media source anyway.The unusual part is that his craziness put him under the microscope like no other POTUS has been since Nixon. All sorts of investigations going on, and they decide to bring box upon box of documents from the White House to one of his properties upon his departure.
That screams a lack of logic or common sense.