Why were everyone wearing trenchcoats and berets? Fans of Sam Kinison or something?

It was the 1980s.

Bug wears a beret because in fashion it is a classic signifier of rebellion. So he has appropriated the look to signal that he is an outsider. This, in turn, is attractive to Tia, who thinks he is a bad boy and is looking for a means to express her own feelings of teenage rebellion.

However to the audience, the beret is ironic. We recognize that Bug is just a poser, a protected white suburban child wearing the costume of a dangerous rebel.

Thus, when Buck commits a terrible crime by assaulting and kidnapping a minor, the audience reacts to it as a very comical moment because we love to see the phony taken down.

The trenchcoats are because it is Chicago and it really fucking cold and they look cool.

If idiots and posers hadn't ruined it, I would be wearing a trenchcoat during the colder months.

Trench coats were popular in the 80s.

I was gonna say this! Walking past the Water Tower in my way to Loyola sucked and I thought it couldn't get any worse. Then I had to walk along the lakefront at Northwestern.