China's economic growth simply isn't sustainable, and that's even if it was true growth. There isn't a top economist alive who believes China's self-reported GDP numbers aren't fudged in one way or another (source: http://www.businessinsider.com/economist-reactions-to-chinas... ). Either one of those situations (an economic slowdown, China fudging the numbers, or the more likely situation: a combination of both) make…
Hedge funds bet against the yuan because right now it is one-sided: meaning they don't need to worry about short squeezes as typical shorters would have to. If yuan were to sharply rise it is pretty certain that PBoC would intervene. This makes the bet against the yuan relatively safe. The only way the PBoC can stop the shorts is to make borrowing cost high. But it is also trying to make the yuan more widely used so it is loathe to do that as well over any extended time.