For some time now, it's been fairly standard among the Left to be Anti-Israel/pro-Palestinian. It's certainly not universal, but it's incredibly widespread. Anti-Israel sentiment has become very common in other liberal groups, even when they have absolutely nothing to do with Middle East politics.
I've really never understood where this bias comes from. Some ascribe it to good, old fashioned Anti-Semitism, but I think that's far too simplistic. Others say that the Left has a strong bias towards the weak and oppressed, and that the Palestinians are, or have been effective at portraying themselves as, weak and oppressed. These, and the few other explanations I've heard, never really seem convincing - the strength, and often virulence, of Anti-Zionism on the Left is hard to account for.
As someone who is so strongly Liberal in almost every way, but also passionately pro-Israel, this anti-Israel bias has always frustrated me, but it's also confused me. Especially when, in so many ways, Israel is actually a model of Leftist, Liberal society. It's not perfect (what country is?), but in so many ways, it embodies the causes espoused by the Left.
This point is made, clearly and strongly, by Jonathan Miller. He argues that Leftist groups really should be supporting Israel, especially since Israel is almost infinitely more likely to support their particular cause than any likely Palestinian State would be. I won't pull any quotes from it - it's too good, so it's worth the click-through.
Economic Justice, Civil Liberties, Gay Rights, Women's Rights and more - Israel is, in many ways, a model country on these issues. Her neighbors are anything but.
As always, there are valid complaints about Israel, some of them very serious. But, for (for example) a LGBT group to support the Palestinians over Israel is ironic, to say the least.
I wish someone could explain that to me.