Well, he was there for 15 years. Perhaps he doesn't want to limit himself by just being able to work in-house for S-E, but I'm a bit sad about this news.
However, he leaves with a Bang, a big bang.
->Which is the FFXIII OST.
As with Uematsu, perhaps he can return to FF without being a Square composer.
Seriously, I can't think of many names that would be able to single-handedly score a main FF game. But he can and he just proved it.
All the best to Hamauzu on his way from here on out.
Good Luck!
Looking forward to: FF Versus XIII / LBP2 / The Last Guardian / Yakuza 4 / ICO Collection