The idiom which is going to be discussed here in this post is: Jump on the bandwagon. Notice the following example to understand its meaning:

Adrian: I thought your dad hated Mrs. Johnson already.
Ted: Well, she’s the manager now, so he decided to jump on the bandwagon.
Adrian: I think most of the other colleagues are like your father.
Ted: I think so, and this is true, unfortunately.

In the example above, Mrs. Johnson wasn’t liked before becoming manager. But once she got the position, many people—including Ted’s father—suddenly changed their attitude toward her. This shift reflects the idiom “jump on the bandwagon”, which describes joining a trend or supporting someone only after they’ve become popular or successful.

Climb on the bandwagon • Jump on the bandwagon • Get on the bandwagon

Oxford:
Join others in doing or supporting something fashionable or likely to be successful.

Cambridge:
To join an activity that has become very popular or change your opinion to share in its success.

Dictionary.com:
To join or support a party or movement that seems assured of success.

The phrase “jump on the bandwagon” dates back to the 19th century. The word “bandwagon” originally referred to a wagon that carried a circus band during parades. It first appeared in print in P.T. Barnum’s 1855 autobiography, The Life of P.T. Barnum.

P.T. Barnum, a famous showman, used these eye-catching bandwagons to attract crowds to his circus. Later, politicians adopted the tactic during campaign parades. Over time, “jumping on the bandwagon” evolved from a literal act into a metaphor for joining anything popular—often for opportunistic reasons.

• Everyone has jumped on the bandwagon to try and stop smoking in the workplace.
• Whenever the World Economic Forum launches a new initiative, many people jump on the bandwagon about globalization—and then do nothing.
• Lots of people jumped on the bandwagon when Croatia’s soccer team reached the 2018 FIFA World Cup final.
• After Donald Trump became U.S. president, many people jumped on the bandwagon against him.
• After she won elections by promising tax cuts, most others jumped on the bandwagon.
• I can’t stand people who just jump on the bandwagon after a win. Where were they during the tough times?
• Beth always jumps on the bandwagon when new fashion trends emerge.
• Mr. Smith wasn’t popular in life, but after his death, many jumped on the bandwagon praising him.
• A: Did you enjoy that movie?
B: Not really, but everybody’s jumping on the bandwagon now that it won the Oscar.