Just watch the price action closely of ETH on Bitmex. You see those big sell orders go away as soon as there is a little bit of buying. They stay away, just until the buying slows down, they pop up again. Just think of it this way, if you have such a big amount you want to sell. Would you throw in a 6 mil sell order while the buys are like average 100/200K? If you really want to sell, would you scare the whole market with your sell orders?
There are 2 reasons why they do it, that is to push the price down to new lows or (what they usually do) push the price down into their buy orders.
I remember the drop from 8K to 6K after the 8.5 high. Back then we could see similar tricks, but was going on at other exchanges as well.
So of course we can't really say what their intention is, but if we don't see a controlled operation throughout several exchanges, we could assume they are just trying to fill up their longs.
But if we see them doing it on several exchanges, than we can assume they are trying to push the price to new lows.