[raz]

It's purely a merchandising standpoint for retail stores. The items are shipped early on in time 1-2 months before the Spring or Fall Waves so that the stores have the items ready to be merchandised for those seasons. The stores do their resets to new planograms/mods on a semi-annual basis, Target/Wal-Mart doing them in January, so receiving new items in November and December allows them to get a healthy stock of items to be merchandised into the new planograms.

The other reason for the Spring wave is the obvious, to have items out for Christmas shopping to maximize profit.