Well, there are famous artists who have done extreme stuff in the past (Van Gogh with the ear thing as an example), and much of today's popular mangakas started off drawing erotic doujinshis or have some very sinful past times (Like Rurouni Kenshin's mangaka and his CP 'hobby').
From what I can guess on how artists and art teachers get 'creepy', part of it is that they were usually exposed to something really unhealthy at very impressionable points in their lifetimes. Not that all artists are 'eccentric' to that extent, of course, but people who have had 'eccentric' lifestyles are prone to be artists of some kind down the line.