Given the small daily trading volume of bitcoin, "that order would be enough to have an impact," said Hunter Horsley, CEO of Bitwise, which is in the process of applying for the first-ever bitcoin exchange-traded fund.
That large purchase of bitcoin could have been triggered by an individual trader or a small group of traders, according to Zennon Kapron, director at financial technology consultancy Kapronasia.
Speaking with CNBC over the phone, Kapron said there were many traders sitting on "significant amounts of bitcoin" who could "very well be behind the movement."
Still, Bitwise's Horsely said: "Crypto is famous for a long history of volatility like this. It's always hard to pin down the true impetus, and often the real source is not obvious."
Other possible reasons for the surge include the crossing of an important technical level at the $4,200 price triggering a stop-loss trade. (In bitcoin and other assets, traders will often set a certain level for when they automatically cover their shorts.)
Asked about the rally's sustainability, Kapron said prices were likely to come back down if there was nothing to support them. And, he added, there was "nothing in particular" that drove the recent price spikes aside from general market demand.
Bitcoin prices have seen a relatively calm 2019 after last year's volatility. The cryptocurrency fell roughly 75 percent in 2018 after climbing to a high of almost $20,000 at the end of 2017.
— CNBC's Kate Rooney contributed to this report.