WordNet records no synonym and no antonym for bandwagon effect. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the phenomenon of a popular trend attracting even greater popularity
“in periods of high merger activity there is a bandwagon effect with more and more firms seeking to engage in takeover activity”
Synonyms, antonyms and the broader and narrower relations for bandwagon effect come from Princeton University’s WordNet. Full sources and licences.