The party van joke comes from internet forum culture, where an unmarked van became shorthand for law enforcement arriving to end an illicit party. On boards like 4chan it was used as a darkly comic warning whenever someone posted something incriminating, and the phrase spread through memes, edits, and soundboards as a quick way to imply that trouble was on its way. It has lasted because the image needs no explanation: a van, an arrival, and an abrupt end to the fun are understood at once.