Basic mathematics really....
It's all to do with twin lobes affecting the needle pointer, confusing it so it points to three random positions each second, oscillations increasing between the head (or tail) with closing distance at a rate proprtional to the coanda effect allowing for the coriolis force. The music being played by the AM radio station is bound to be a hard rock choice by Slim Dusty or Dolly Parton. Ha, I knew I'd get back to twin lobes somehow...