Here's the artist's comment.

PixivからTINAMIへ創作の場所を移動します。アカウントは消さずに製作物の報告、イラコンへの参加に使用します。TINAMIでもよければよろしくお願いします。

It must be related to Pixiv artists making a mass migration to Tinami (originally an artist database, but now is like Pixiv in terms of function).

Pixiv issued a statement regarding what happened.
For those less familiar with moon, ANN article.