Their D zone play is trash.
They were able to cover it up the last two years with dominant possesion and dominant goaltending, respectively. This year, however, their losses in the bottom 6 are glaring and it's affected the overall play of their forward group, not to mention that Murray can only do so much. He's done a LOT of bailing out, which is a big reason why the Penguins are still in a decent position standings wise.
As of right now, the Pens don't look like a playoff team in this division. That said, I think they'll bounce back. Rutherford will find a way to get some quality depth for his trash and they'll be poised to make a run.
Oh, and Letang will need to wake the f*** up as well.