Pure speculation. This guy got the outtakes by hacking FOX and sent them to Media Matters, who in turn gave them to to other liberal outlets in order to embarrass both Tucker and FOX. FOX may have figured out he took them and got the FBI to investigate since FOX was being blamed for the leaks.
after running this part of the story I get this: "by receiving them from Irena Briganti under plausibly deniable means"