If you Jump On The Bandwagon then you join in, often uninvited, an already successful venture to gain some sort of personal benefit. Travelling bands in the Deep South of America used to perform on their wagons in front of rallies. The bands would normally be one of the main highlights of an event, and so it was common for political or religious leaders to climb up onto the wagon and interrupt the music (occasionally with prior agreement) to get themselves a captive audience. Crowds would usually tolerate because they’d know that the musicians would be playing again soon. The practice had been going for over a hundred years before it was first recorded during William Jennings Bryan's presidential campaign in the early 1900s.