Apparently himself according to Cajun.
I admit it's unlikely, but how do you know it actually isn't him? It's not like we constantly hear Pardew yelling at him to stay back, and afaik he hasn't really mentioned that Cabaye needs to sit back more and help the defence in any press conferences. Maybe Cabaye himself feels he needs to help the defence more, or simply feels like he is more useful around that area of the pitch. Just seems to me like its very easy to blame Pardew without actually having much knowledge of the conversations that go on during training, in the changing rooms etc.