Its a ghost, no question. You know who to call. (ghostbusters)
No but in all seriousness, chance is a strange thing. One would think if you roll a die you would most likely not roll the same number 10 times in a row. or flip a quarter and get heads every time, but I've had it happen. Thats why its called chance. I'd guess that a chance RF signal just happened to screw with your tape recorder.
But I can't say with any certainty what happened anymore than anyone else. Believe what you like. Who knows maybe there really are ghosts out there. We only understand a very small fraction of how this universe works. I know theres lots of things that we are sure of today that in the future will seem as silly as being sure that the Earth is flat.