Originally Posted by Apokx

Once Kel'Thuzad was whole again, Arthas led the Scourge south to Dalaran. There the lich would obtain the powerful spellbook of Medivh, and use it to summon Archimonde back into the world. From that point on, Archimonde himself would begin the Legion's final invasion. Not even the wizards of the Kirin Tor could stop Arthas' forces from stealing Medivh's book, and soon Kel'Thuzad had all he needed to perform his spell. After ten thousand years, the mighty demon Archimonde and his host emerged once again upon the world of Azeroth.

Archimonde destroyed the city.The bubble is either to keep beings out while the Wizards rebuild, or it's to keep something inside.I dont think Blizzard has explained why exactly the bubble is there..yet