I think that the whole Jewish/Progressive tie goes back to the Old Testament. In it, Jews were commanded to look after the poor, the widowed, the orphaned, the stranger in their country and so on. I believe some Jews that take the side of Democrats are actually following that commandment. After all, the Democrats have passed themselves off as the "Party of the People" for several generations...
Many Jews also have a very strong intellectual background as well. Unfortunately, that intellectual acheivement has sometimes overshadowed the relgion/faith part of the Jewish identity. Often times, Jews have gotten involved in far left wing causes, even to the point of denying the existence of God.
David Horowitz once wrote something about this. Jews saw themselves as being outcasts from society and discrimated against by the mainstream. Thus, their attitude toward "the establishment" is understandable, many want to change the fabric of the society that they feel has been discriminating against them for so long. That is, those Jews who still feel outcast.
However, many Jews are conservative, Ben Stein, David Horowitz are two that come to mind. In their case, they don't see themselves as being outcast, but see the American form of government as the Jews' best chance at having a better life not only for themselves, but for their children, attaining equality and for once, being able to practice their religion without fear.