Fundamentals
The common census is China economy growth will slow down significantly, due to the slow down from export (trouble in US economy); rising inflation (food, oil etc.)
The rights of minority shareholders are also not protected as well as mature market, because in some cases the management cooked up the accounting books, and get away from it.
The flaws in market itself
Recent arrest of former vice head of China securities regulatory commission (Wang Yi): people fear this is not an isolated event, and bigger fish will be caught as this thing unravels. Insider trading was rampant and still is prevalent in China. Insider trading reduces the confidence of long term investors, people just want to make quick money and run.
A thousand words 