Meh, it just slow. No big games, no price reduction, what would you expect. There is no exceptional reason to run out and buy a Wii in Japan right now.
From what I can see in my separate excel file, all consoles have dropped of at a similar rate.
Last month percentage drop:
Wii = 29.1%
360 = 28.5%
PS3 = 4.4%
Projected this month
Wii = 47%
360 = 34%
PS3 = 33%
Now, 360's may be less when Halo 3 releases. I am not sure how much it will affect Japan. Either way, Wii's has had the most significant drop, but I would argue that this is simply due to lack of any big title releases and there aren't really any games being released until Nov 1 (Super Mario Galaxy). Next in Japan is Opoona on Sept 27th, then literally no games until Nov 1. Expect the Wii sales to continue to have slight drops until then.