The Brief: To bandwagon means to take part in a trend once it becomes popular.

DEEP DIVE
The term bandwagon is commonly used to ridicule a person for only liking something because it is mainstream or successful.
For example, a hardcore Star Wars fan might say that people are just now hopping on the Yoda bandwagon.
Don’t think for one second I’m not on this bandwagon @starwars pic.twitter.com/51aYX2MxzG
— Katie Cook (@katiecandraw) November 21, 2019
Bandwagon is also used in sports to accuse “die-hard fans” of supporting whichever team is currently playing the best.
The thing about the “Warriors fans used to be Laker fans now they’re going back” thing is that it’s just saying Laker fans are bandwagon. They left when their team was bad and came back when good. Has nothing to do with Warrior fans. https://t.co/o8MG5P9cuY
— DVS (@DVSWhatItTweets) November 21, 2019