What is a Sketch Again is a tag used for illustrations, claimed by the person who submits it, to be a sketch when it obviously does not look like it.
Yeah, it surely does not look like one
While such artworks are usually thought to be artworks that are obviously beyond the scope of a sketch, the artist might thought of them as one (usually known by their work title or their work description). Since their definition can be completely subjective, there is no deny that what they submitted would be a sketch.
Even so, for sketches that obviously look like one are also tagged that way due to their high quality. Rather, works like this appear more often recently.
See also : This is...a Sketch... (落書き・・・だと・・・)