In relation to something or someone that suddenly becomes overly popularised and gathers a huge following of people who don't know what they're talking about (usually due to the media). So named after
Spider-Man's Uncle Ben. Nobody cared who he was while he walked home in the streets, but the second he gets shot a crowd of people who are unimportant to him suddenly gather thinking they are helping when they're actually just in the way.
"Your girlfriend is a huge Avengers Endgame fangirl, even though she's never seen any of the previous films? Wow, Such a gunshot crowder"