Witch hunts happen when we face problems that we do not understand and/or problems which seem to threaten our very way of life. In such situations, we try to find scapegoats to blame for our problems.
A liberal might say that an example of thisis the way that we look at illegal immigrants today. We know that our economy is struggling and that American society is becoming less like it was in the old days. We do not know why this is happening, so we blame it on illegal immigrants.
A conservative might make the same argument about rich people. They might say that we blame the rich (bankers, Wall Street, etc) for our problems because we don't understand them.
None of these things has reached the level of witch hunt yet, but the idea is the same -- when we encounter problems that scare us, we look for scapegoats. We generally pick people who are unpopular and use them as our scapegoats. This can become the basis for a witch hunt.