The dun dun dun sting comes from the stock dramatic cues used for decades in radio, television and film melodramas to underline a revelation or cliffhanger. As an enlatados, or canned, effect it was shared as a ready made library sound rather than a clip from one specific scene. Online editors adopted that same cue for memes and short videos, using its old fashioned seriousness as a joke in itself. It remains popular because three notes communicate shock, suspense and a punchline without needing any words or explanation at all.