For a while now, it has been known that the Chinese exchanges, particularly OKCoin, are involved in faking trading volume. Bobby Lee of BTCC has spoken openly about this. The ex-CTO of OKCoin admitted as much in a statement made after leaving the company. The famous "volumizer" running on their futures platform is known to amuse traders in slow times of market activity, as seen below:
In the chart below, note how during periods of low volatility, multi-thousands of coins are exchanged, exceeding the volume during a significant price change.
Bitstamp had offered 0% fees after they were hacked earlier in the year, which would make some sense. Their marketmakers could have a special deal where they receive a rebate for volume delivered. This is all speculation among traders, and nobody knows for certain. But there's enough strange action going on with their price action and volume changes that it has been arousing suspicion.