This is just a fan theory. (spoilers)
An anonymous writer from the school tells everyone that Hannah’s death wasn’t a suicide but a murder. I would make sense as some of the things she mentioned on the tapes are incriminating like Bryce and her in the hot tub or him and Jessica. Therefore someone like Montgomery would cover Bryce’s tracks and save his ass. But little known to Bryce the tape would still be sent out.
This makes sense as in the show Hannah kills herself but using the razor blades at the pharmacy. In the novel she is rushed to hospital but it’s to late. Finally is the book clay mentioned she took pills. The reason why all 3 explanations are different is that all of them are liars Montgomery and Bryce started.