I think it's probably more accurate to say that everyone felt they were supposed to be familiar with the major wine varieties so all had picked up the terms and desperately clung to a few half-earned opinions because they didn't want to appear unsophisticated. But I think most people didn't actually have particularly strongly held or individually acquired wine preferences. (It's well known that even professionals can often not correctly identify what variety or even what color wine they're drinking.)
This is a social situation that's ripe for a radical opinion swing. Because everyone is trying to look like they know what everyone else knows but there's little "ground truth" (i.e. actual personal opinions) slowing down the change. So when a movie like Sideways comes out with a character who clearly cares deeply about wine and knows a lot and expresses a strong opinion. Well, I guess merlot's not cool anymore.