I think this is a ridiculous rumour started by the Clintons themselves.
They hope the Vast Right Wing Conspiracy will run with it, and thus generate loads of sympathy votes for Hillary both from those who foolish enough to believe that she is a victim of Bill Clinton and more importantly, from those who are foolish enough to believe that she is a victim of the Vast Right Wing Conspiracy.
There really is no other explanation. The rape accusation is competely ridiculous. So ridiculous in fact, that it had to have come from the Clintons themselves... No reporter would trust any other source.