Why the change? We have no idea. It could be a temporary change while Blizzard works on an issue with the old login screen -- or perhaps it is the end of roaring, animated dragons when you log in to the game. The particular art used is gorgeous, obviously, and it's one of my favorite shots of Deathwing.
The reasoning behind the change will most likely come out soon. Tin foil hat speculation puts it anywhere from people's having issues with the login screen and just not loving roaring dragons (even after the toning down of said roaring) to using a new login screen that reflects the next tier of raiding. We don't know! My guess is that we will know soon, however.
The news is already rolling out for the upcoming WoW Patch 4.2! Preview the new Firelands raid, marvel at the new legendary staff, and get the inside scoop on new quest hubs -- plus new Tier 12 armor!