"Bandwagon propaganda" is a type of advertising that attempts to draw people to a specific cause by giving the impression that many others have already joined and are happier or better off for doing so. It uses societal pressures to play on several basic elements of human nature.
Bandwagon propaganda gets its name from the vehicle used to transport bands or other important figures during a parade. When a particularly popular group or figure was seen in the parade, fans would literally jump on the bandwagon to join them. Thus the term "bandwagon" became synonymous with the kind of popular trend or activity that attracts followers suddenly and in large numbers. In terms of advertising, the term is meant to express the same feeling of excitement and admiration that motivates people to jump on a bandwagon.
A common example of bandwagon propaganda is the celebrity endorsement. Most people associate popular celebrities with happiness and an elite echelon of society. When they see a celebrity using a certain product or supporting a certain political candidate, they want to emulate the celebrity in an attempt to become more like him. Hence, they support the same political candidates and purchase the same products. The process is based around humanity's natural desire to become better and to be associated with the best products and most successful people possible.