I think a lot of it has to do with the narrative that Israel vs. Palestine is depicted akin to "white people vs. colored people", "colonizer vs. colonized", and "European vs. Asians/Africans". While both the Jews and the Arabs are minority populations in the West, the Jews as a people have integrated to the point where some see them as incredibly privileged rather than oppressed as people associate with Palestinians, Arabs and/or Muslims. Because of the tropes a lot of people who are liberal use in other conflicts and situations, people look at the Arab-Israel conflict and make assumptions that aren't so nuanced.
Because they're viewing this with the concerns to protect minority populations (from a Western perspective), that just makes them want to be pro-Palestine rather than be pro-Israel. Some of this may be genuine, but I feel like a lot of people just refer to the "social stack" and just view protecting Muslims above other groups (such as Jews) or protecting the brown people over white people (despite the fact that both Palestinians and Israelis have populations that largely can pass as one another).
Now, not all liberals are so anti-Israel and a few do criticize Palestinian society for their conservative and right wing stances (particularly the problematic ones, especially regarding LGBT and women's issues). Still, there are a lot of liberals who honestly know nothing about Israel yet go into condemnation of it based on superficial vantage points and misunderstandings.