A bandwagon is a trend that is so cool everyone wants to get in on it. If you start wearing a flowerpot on your head because everyone else is, you’ve jumped on a strange fashion bandwagon.
Originally, a bandwagon was a large wagon that did indeed carry a band. Now it’s an idea — people jump on the bandwagon when they hop on a trend. This word can be negative because it’s what people do only because it’s trendy. Longtime fans of the Boston Red Sox may resent new fans who like the team because it's become the "in thing" to do; the old fans would accuse them of jumping on the bandwagon.