Sudden assembly that some find entertaining

• Apparently spontaneous public gathering
• Group behind a public stunt
• Outlying motorway cuts through ruined Exeter
• Seemingly spontaneous public assembly
• Virtually organized crowd
• Seemingly impromptu public performance
• Uncoordinated dance group?
• Spontaneous public gathering

Wiktionary Word definitions in Wiktionary
n. (alternative spelling of flash mob English)