Injuries to the Centres.
Bozak being out made McClement jump around, even sitting on the 1st line for a stretch. That meant McClement was overworked on even-strength ice time, so on the PK he wasn't able to play effectively.
Bolland is also a key part of the PK unit, and has been out most of the season.
Add to that that Bozak wasn't there either, and even once back in the lineup, his faceoff % has dropped way down since the injury obviously is still affecting his timing.
Those would be three major components that I think account for a good chunk of the increase in PK GA.
Other factors: Gunnarsson and Fraser having terrible seasons. Those guys were defensive cogs, although things have improved greatly since the addition of Gleason, in that respect.
Beyond all that, I'm not sure. Perhaps the lack of success has also had an impact mentally, further weakening the PK.