(to jump on the) bandwagon
Defined: to offer support for something that is popular; to become active in something that has recently become very popular
Example: Instead of simply exercising for 20 minutes three times a week, Maggie chose to jump on the bandwagon of every diet that came along.
A bandwagon was literally a wagon that carried a musical band during a parade. In these parades, it was the practice of a person running for president or other public office to ride on the wagon and address the public from it. During these parades, it was not uncommon for a person who was trying to advance their own career (usually political) to jump on the wagon to show their support.
The phrase started out as "hop on the band wagon" and over time the word "hop" was replaced by "jump." The term bandwagon has been around for some time, but it appears to have been first used in a political sense in 1884. In that year, Puck magazine published a cartoon of the presidential candidates of that year riding a bandwagon.