It's going to be tough answering this for all possibilities, and if there's any information on why Kuma Kuma Kuma Bear in particular did it then it will be hard to find and in Japanese, but as a broad answer it's because any anime adaptation will have two audiences:
The people who are familiar with the original work; and
The people who aren't.
The people ...
I doubt there is an actual name for this but it's likely a trope that derived from older manga works., most notably that of Fujio Akatsuka. It's best described as an "afterimage" of movement.
There are many names for such, like "ghost feet", but there's not any real agree upon terms for this trope.
From this book about super-deformed ...