To follow a group that has a large and growing number of followers.
A bandwagon is literally a wagon which carries the band in a parade. The phrase “jump on the bandwagon” first appeared in American politics in 1848 when Dan Rice, a famous and popular circus clown of the time, used his bandwagon and its music to gain attention for campaign appearances. As campaigns became more successful, more politicians strove for a seat on the bandwagon, hoping to be associated with the success.
However, by William Jennings Bryan’s 1900 presidential campaign, the term was used in a derogatory way, implying that people were associating themselves with the success without considering what they associated themselves with.