He/She is selling 40 000/50 000 Nano daily (some say it is just moving them), but it seems that coins are selling on Kucoin exchange. I do not say it is only thing that push price down, there is also BTC price which affect the whole market - but this seller is helping that Nano is losing value day by day. There is over 1 million Nano on that account, and it seems that all will be sold. Maybe this is not a such bad news, cheap coins will get new owners.
He bought from captcha week hands for a few cents at 2017. Actually he is almost sold out , 300K NANO left and I guess he will left some for long term.