The Radar story claims that when Bubba left Sirius for a return to terrestrial radio, he "left behind an unpaid staff." One former staff member was able to get their hands on the tape and that's what led to this entire mess.
The report claims that Bubba had shown the tape to a "number of people" and that Hulk Hogan was first aware of it three years ago, well before it began making the rounds for a potential sale. Radar claims that "PR forces" within Hogan's camp purposely allowed the story to grow as a way to get attention for Hogan.
How they may have gotten the tape from Bubba, however, and who the staffer is, are not covered in the Radar story. With Hulk Hogan seeking to press charges, that will likely come out sooner than later.