Customers wouldn't be happy about BitMex decision.
If BitMex could managed sold bch at the peak, maybe their customers won't complain.
Seems they do not like hard fork at all, don't want to credit their customers either with bch or b2x.
And have tried to dumped bch after it pumped significantly over $1900 few days ago.
Not necessarily. Remember that people who had BTC on the exchange at the time did not receive their forked coins. This means that some of them were not given a chance to sell early, when the price was much lower than it is now. This means that the exchange isn't doing anything wrong. On the contrary, it's selling BCC at current, high prices.
As for the crash we don't know how much BCC the exchange controls. Also, they don't have to sell it all at once but do it slow. It's not like they're on fire.