I don't think "someone" is buying this much of bitcoin. It is probably a group of deep-pocket investors who are overtly bullish about the bitcoin's future, are buying! However, I am still not able to find any credible source of your information. If someone had bought it, that would have become a news in crypto news websites, but apparently no one took notice! Most probably it is a group of investors who bought bitcoin but it seems like there are also sellers who sold this amount of bitcoins at the current price level. Big reason to worry?