To become part of the newest activity because many other people are.

Barry's boss has nothing to say (or ad) - unless his boss says anything and then he quickly jumps on the badwagon and spreads the good news to everyone else.

Many years ago candidates for political offices in the United States often rode through town in horse-drawn wagons on which a band was playing music to attract a crowd. If the candidate was popular, people would jump up onto his bandwagon to show their support. Today we say that people who are getting involved in any activity that looks like it’s going to succeed are jumping on the bandwagon.