There are plenty of reasons for that.
-They downgraded their center position by trading away Bynum. Granted, Bynum wouldn't even be able to play, so, they can feel they were lucky were this season.
-Gasol hasn't been as good as he was in the past seasons. As soft as he has always been, but he was a whole lot more efficient. He was derailed by injuries, as well.
-Nash isn't the one we know anymore. He can't carry the team offense alone, and when he can't do that, his defensive liabilities become more critical.
So, I'm not one bit surprised by their current state. I expected them to be a couple of games above .500, to be fair, but a borderline play-off team, that's it.