In the book Moriarty had two twins of himself (probably
) also many fans have started theorizing : many believe he is not actually back , and it was just done by someone (Mycroft ? Anderson Fan Club ? anyone else ? ) to stop Sherlock's exile . Also it may happen that sherlock didn't fully trace Moriarity's web and some of his most loyal ones still remain , so like sherlock's 13 plans on the roof , Moriarity also would have some plans of his own , In case sherlock didnt die . As the news spread that he has survived the remaining moriarity's men laid out the plan and after some time posted that video as hacking various places and laying all the stuff for the plan to work would take time ... To be honest I dont believe in any of this because I want Moriarity back , I missed him.