Movies get schedule-changed all the time.... so much depends on what space is available in the theaters, what other movies the studio is releasing at the same time and making sure that they don't release similar movies (i.e. 2 sci-fi movies) at the same time.

But more than likely the sfx shots may have slipped schedule. And no doubt problems with the writer's strike going on... theoretically, they should not have been doing re-writes/changes/updates to the script during the strike.. ha, ha...right...)