Two major factors:
Injuries, particularly to Chris Martin, who has missed the last couple of months apart from the odd game when he hasn't been fit. Massive cock up by McClaren not having at least a low rent option as a back up as he is so pivotal to the way the play, Bent has scored goals but does nothing for the overall pattern of play of the team. They also have 3 holding midfielders in Thorne, Eustace and Mascarell all who have been out for at least the last couple of months hence why they are conceding goals as they don't have anyone to protect the defence and why you get desperate selections like Stephen Warnock playing there today.
The other big issue is the open secret that McClaren is off to Newcastle (or at least was, they are now serious relegation candidates so not sure how that would play out), which has been massively destabilising.