It's not just because of Monster Hunter. The 360 slim was also released in Japan that same week. About 15,000 of the 360 consoles sold that week were slims, according to MS Japan. On top of that, the older 360 consoles had a ~$50 price drop. The sales aren't that impressive, all things considered. But it might be a supply issue.
Palmtop Tiger said:
I heard this Monster Hunter has a $15 a month subscription fee. Yikes.
It's a port of the Monster Hunter game available on PC, which also has a $15/month subscription fee. The differences here are that the 360 port includes all of the expansion packs released up to this point and comes with 12 months of XBL Gold as an extra. Unfortunately, the PC and 360 versions are running on separate servers.