I know some artists are modest about having their art being spread around, but in Yummy's case, he doesn't want any of his works from here to spread for the obvious reason of pedo content. Finding these works from anywhere else can easily connect them to him, and the less of his work that's out there, the better. It's not hard to connect the dots.
Micro didn't want people posting her old art because of how bad they are compared to her newer stuff. She specifically said as much long ago. It's out of shamefulness, not for the purpose of going into hiding.
All things considered, it's more likely to have it be legal issues than anything else, but unless Yummy decides to share more details, we can only speculate.