To start out with, I do not know the answer. But I would love to hear theories.
Historically, the Left has been very pro-Israel and pro-Jewish. But of late (what, in the last 4 years??) the Left has turned against both Israel, and to a lesser degree, the Jews; and has turned more pro-Islamic. Witness the Berkeley thread etc.
One guess I have is that Israel is being ruled by "conservatives" and that ideology is playing its hand. Would the Left in this country and in Europe be against Israel and the Jews if Barak or Peres were President?
I don't think one can say that the Left has always been anti-semitic, and that it is just coming out. Something has changed. Is it that the Right supports Israel and therefore the Left must not support it? I don't know.