If the interwebs and rumours are to be believed (and it kinda makes sense),
GW had these new codexes and models ready for some time now. I guess they are releasing in waves because of the reasons -Loki- wrote.
I'm very happy
tbh. I only started playing in the 6th edition and this new way of doing things is really good, allowing people with several armies to save up some money before the release of another codex and units they really want. Not only that, soon there won't be more 6 (?) year old codex or some crap like that. After Eldar, I dearly hope Black Templars get an update, I think they are the ones with the oldest codex after Eldar? (And
SoB better get an official codex

)