Ok so generally speaking we have no idea how blizzard is making it up for us - but here's my idea
(B)lizzard : "Ok we introduce Pandaria Continent"
(P)layers : Ok....But where the hell it was all the time?
A short brake in info from blizzard (some "high" sessions, drunking)
B : So...all the time it was cloaked in "mists"
P : Ok...Why mist? How it began?
A short brake in info from blizzard (some "high" sessions, drunking)
B : Oh it was because of the "last emperor of Pandarens" - throw random Chineese name - we'll explain it "soon"
P : Ok...
A short brake in info from blizzard (some "high" sessions, drunking)
B: So we decided that the emperor farted really hard and it covered whole continent in a mysterious mist. But when Deathwing flew over it he cleansed the fart with his wings which damaged him enough that we could later kill him with an old artefact...
P: Yay!! Superb!! IT'S SUCH AN AWSOME IDEA! It's 100% true
... Hope u see my pattern here?
They throw at us what they want and ppl believe them... make it as it would be some kind of a weird "true". I know it's a game... but their ideas are a little bit strange sometimes (now more then in the past)... I would prefer to leave the mist as a mystery and never know about it. Anything they will think about now will be as good as my idea of a Pandaren Emperor rotten dinner.