Wizards pointed it out themselves, they've been equating the last few blocks as horror/disaster movies. Alara shows a plane-wide destruction (unless my memory's off), Zendikar has the Eldrazi roaming around, and New Phyrexia is...well, New Phyrexia. An interesting thing to note about Rise of the Eldrazi, each of the artwork for at least the colorless Eldrazi takes place during the daytime.