면 따르다

if X, follow along, copy it

A conditional "-(으)면" plus 따르다. Seven of these eight lines use the "follow, copy" sense: do you sing along when the music comes on, people who cry when others cry, if this one buys they buy too, if trust comes back the price follows it up. It is the copying-what-others-do sense, which is why it turns up in talk about crowds and markets. The eighth line uses the other 따르다, a homonym meaning "pour": the custom of refilling someone's glass once it's empty.