"This is a lethal combination: personal humiliation plus the comparative acceptability of suicide in Japanese culture."
I wonder if there is another element evolved here as well, particularly differences in sensory perception and stimulus intensity between Japanese and European cultures as was demonstrated in various experiments.
Also, when it comes to younger population, especially in the recent years, there may be several additional factors here, like "the copycat effect" (which also works in waves of murders), fascination with fantasy or fictional world - being intensely inspired by or strongly identifying with (like computer games, cosplay, rich culture of myths and fantastic stories with hyperdimentonal themes). It's like it is not only honorable to put an end to one's life, but it is honorable to end it in a specific way or place that is tied to a specific story or legend. To be connected to something, or totally escape into something seemingly more meaningful and fantastic that transcends reality, because reality is harsh and devoid of any such wonder.
Besides, take a male dominated culture where one fourth of all suicides are financially motivated and are not condemned but seen as a responsible way out, and where social rules and expectations are so high and demanding. Couple it with different and intense perception of reality that has to be bottled up to comply with the said demands, and you have a culture that will seek an outlet in all kind of, sometimes intense or unique, ways.
My knowledge of Japanese culture is pretty shallow, so it is possible that all the above is way off, but my impression is that on one hand there is an emphasis on duty, honor and responsibility, on the other hand, on fantasy and myths. Maybe it was different before (watched several Akira Kurosawa movies, and they have an amazing depth of thought and emotion there!), but perhaps these days their cultural norms including emotions and emotional needs are being distorted and manipulated. Notice how their achievements are focused on technology that perpetrates the fantasy world and further disconnect from emotions and reality. It's like there is some kind of generations long social experiment going on. And people react in Pavlovian (transmarginal inhibition) manner.