My understanding is that editor/publisher Bill Olver is responding to reader comment and demand, in the hope and expectation that three different magazines that hew more closely to traditional genre boundaries will sell better and find a more reliable subscription base. I hope he's right, and I don't know enough about the business of publishing to second-guess. From my perspective, if it results in my story being read by more people, and by more receptive people, so much the better. (That it means my story's publication will be pushed back until November 2014 is, in the grand scheme of things, a minor annoyance.)
There is one aspect, though, in which I find it disappointing. As both a writer and a reader I sought out a multi-genre publication for this story. As a reader, because the set of what I enjoy reading does not map isomorphically to any known genre. And as a writer, because there is an aspect of this particular story that, I believe, can be read in at least two ways, depending on the frame of expectations the reader brings to it. In many cases, such framing is provided by genre. However, since M does still pair horror and mystery, two genres which I believe to have very different ontologies, I can still hope that some readers will be suitably confused.
As Big Pulp undergoes its mitosis into three distinct entities, I hope the ecological niche it leaves behind, of multi-, pan- or non-genred magazines, soon finds a new occupant. In the meantime, I hope you are sufficiently intrigued by my Jumping Frenchmen to feel the urge to subscribe to M. Do it!