bandwagoner

noun

band·wag·on·er ˈband-ˌwa-gə-nər

variants or bandwagoneer

¦band-ˌwa-gə-¦nir

plural bandwagoners or bandwagoneers

chiefly US

: a person who takes part in or becomes enthusiastic about something only when it is popular or fashionable

… ugly performances through December and the first half of January caused more than a few bandwagoners to jump ship …—

Kevin Greenstein, New York Sun, 6 Feb. 2008

… Hawks fans bellyached about the affluent bandwagoners who are scooping up tickets …—

Dan McNeil, Chicago Tribune, 28 May 2010

Though they are sometimes derided as shameless bandwagoners who leap into and out of stocks with abandon—increasing volatility in the process—it's not always a fair charge: Some momentum investors actually hold stocks for relatively long periods.—

Melanie Warner, Fortune, 9 Dec. 1996

The original innovators have moved on, and all that remains are bandwagoneers pumping out an astounding amount of identical-looking product …—

Patricia Calhoun and Justin F. Farrar, Houston Press, 31 Aug. 2006