This kind of slip sound comes from classic cartoon sound design, where a slide whistle and a soft impact were paired to make a fall read as funny rather than painful. The same short effect has been reused countless times in online videos, memes and animations because it is instantly understood without explanation. Creators reach for it for slapstick edits, fail compilations and playful transitions, and its half-second length makes it easy to drop into fast-paced clips without slowing the joke down.