OP isn't original at all, not to be degrading, but whenever I go to see Japanese artist, there always seem to be a lot of metalheads. They often listen to J-pop and watch anime too, so it seems to be a somewhat natural proces. I too was a metalhead, well somewhat, never liked "true" metal like Ozzy, et al., and I started listening Japanese music too. It's most likely that because of being a metalhead, who always seem to be more open to other genres, that you gave this a try.