Microsoft seems to have a strategy of releasing core games at about the same time Sony does.
Killzone 2 February - Halo Wars February
Halo ODST Fall - God of War III Fall
Heavy Rain Christmas - Alan Wake Christmas
GT5 Christmas - Forza 3 Christmas
Of course anything can happen, but, I think that Microsoft and Sony are competing with each other and trying to make sure the other does not release something unanswered by the other.