It's days like Monday that reassure Tony Hann he was right to avoid stocks in mainland China.

The severity of an 8.5 percent drop in the Shanghai Composite Index is bad enough, but what irks him the most is not knowing why it tumbled so much. In a market where unprecedented intervention has made government money one of the biggest drivers of share prices, authorities aren't transparent enough for investors to make informed decisions, said Hann, the head of emerging markets at Blackfriars Asset Management Ltd.

Monday's plunge was all the more surprising because it followed a government rescue package that had helped drive a 16 percent rally since July 8. That support appeared to vanish without warning, leaving analysts guessing whether authorities shifted their policy stance or just got overwhelmed by a flood of sell orders. After the close of trading, the securities regulator denied speculation that the government has exited the stock market.