According to an unnamed Kodansha editor, he blamed unauthorized foreign manga scans for the rise in scans in Japan. To quote directly,
[b]efore, it was mostly non-Japanese kids posting and translating manga. But the kids in Japan caught on, and now all kinds of manga are available for free as soon as they hit the shelves [in Japan].
Whether such as claim is true or not is not known but news of arrests due to piracy has been more prolific of late and to cite a paragraph from ANN, “Yukari Shiina, an employee at the World-Manga.com agency, …does not buy “‘scanlation groups’ argument that they promote manga in general. It might be true with some obscure titles, but it can’t be with hits such as Naruto.””
For the original article, stop by Daily Yomiuri Online.