This happens a lot if you kill the boss too early. Happens in Bastion of Twilight a lot if you killed the council boss too early as well.
Basically the doors take forever to close, and if you kill the boss before they “close”, they won’t register that the boss is dead. So they remain closed forever.
So yeah, listen to the Pandaren and slooooooooooooooooooooooooow down.