Due to your valiant efforts in protecting the key invasion cities, the four boss portals are now 'permanently' open. If you need anything from inside the portals, or maybe you dropped your keys in there, now is the time to go through and get them.
Initial reports from shaman in the area that were available at time of publishing suggest that this is hinting at something far more sinister. One such shaman, a one 'Krunkmaster' from Proudmoore was quoted as saying "This is hinting at something far more sinister."
You'll want to stay tuned to the World of Warcraft community site for the latest developments on this and other events.