Possible Clues:

• Gathering organized via social media
• Crowd doing a sudden performance
• Unexpected gathering of streakers?
• Group behind a public stunt
• Spontaneous crowd performance
• Loose mass seizing quiet moment — to perform a prearranged public act?
• Sudden assembly that some find entertaining
• Quiet moment seized by fat people gathered by phone
• Spontaneous gathering, and a hint to the starts of 17-, 23-, 37- and 48-Across
• Group that assembles suddenly in public, performs and disperses