Likely Theoretical Reasons (in descending order):
1.) Western Digi is coming out with some unannounced model of new high-volume consumer (7200 RPM) hard drives, possibly based on the platters in their new 1.0 TB drive. However, platter production is behind due to bugs. (Remember, their upcoming 1.0 TB drive will be Western Digi's first drive using perpendicular recording technology!)
The result is that management screwed up and miscalculated the cutoff date for existing drive models too early. Now they have a product availability gap. The presumed production problem could also be related to some other production problem besides platters (like #3).
2.) WD won a big OEM order (or two) from someone -- retail channel be damned.
3.) WD is a bit cash-short after buying Komag and can't afford to pay expedited material costs for manufacturing existing and upcoming drive models.
4.) Dubya Dee fired the secretary that took care of ordering paper, paperclips, and aluminum for manufacturing hard drives.
5.) Western Digital is breaking up into four separate units: Western Digital, Eastern Digital, Northern Digital, and Southern Digital -- with Central Digital acting as the parent corporation.