It occurs commonly in chinese exchanges.They generate fake buy and sell orders to show increased trade volume.Chinese exchanges always try to dump the bitcoin price whereas other exchanges try to show bitcoin price as increased.
OKCoin and Huobi do have pretty exaggerated figures at times that they look artificial and faked. However there is no proof to back them up at this point in time.
I doubt that we will ever find any proof of them either faking volumes or not faking volumes because they are the operators of the site and they are able to provide any information whether false or true to the public, meaning that we can’t accurate decide.
But considering that the chinese government is putting on more restrictions, and keeping an eye actively on these trading places, i would say that there is less chance of volume faking happening.