Sometimes authors who wants their series getting serialized and have no intention drawing it by themselves or wanting for their work to have proper art, can hire artists. so yeah which may cost them a lot. Just imagine illustrators that sometimes open up commission works, and for good artists slots always gets filled up. After art was commissioned, means person who paid for that art, has all(or negotiated) rights to use it however they want. Like having an ownership rights for the art.
And second option is, when there is author and writer collaboration, usually they divide the responses, such as author being responsible for story dynamic, plots characters personalities and at the same time they take the responsibility of promoting it as well as protecting it from non profit leaking. While artists fully focuses on artistic aspect of the work. So it is kinda understandable why YD is protecting his/er work: it costed a lot for him/er or it is his/er obligation to protect it from being leaked for free.
And it doesn't mean artists doesn't care, they just can't have time and nerves for this sht fights.
So someone else entirely drew the manga? Not YD? Since I never read the thing I just assumed that YD was the artist, since she’s acting so hostile over this. Nevertheless, from what I can see (the cover & snippets online) the art was pretty. I wonder how the artist feels about this situation & how YD is acting. That is, if what I learned was true