Now if you've ever been annoyed by someone asking your sign and only to respond by sizing you up, you're going to like this. Astrology's compared to racism because signs are determined by when and where we're born, neither of which we have control over, and then others use it to formulate opinions (prejudices) about us. Once astrology zealots know our sign, they look for behaviors — say impatience in Aries or emotional instability in Cancers — to confirm their hypothesis.
The difference, of course, is in how it's used. Since astrology hasn't waged any wars to my knowledge, I'd say it's not quite the same. But when followed excessively, is it a form of prejudice?