It's been hard out there for a bitcoin trader.



The cryptocurrency has plunged nearly 30 percent in the last month, and more than 50 percent this year. But Tom Lee of Fundstrat may have just uncovered the reason behind recent weakness — or at least part of it.



In a recent note, Lee, the firm's head of research, wrote that since the launch of Cboe bitcoin futures in December, prices have plunged leading up to expiration.



"Bitcoin sees dramatic price changes around CBOE futures expirations. This was something flagged by Justin Saslaw at Raptor Group. We compiled some of the data and this indeed seems to be true," Lee wrote Thursday. "Overall, bitcoin has fallen 18 percent in the 10 days prior to CBOE contract expiration."



Lee added the two exceptions were in February when prices ran up nearly 15 percent into expiration (he attributes this to tax selling), and April when prices were up 16 percent.



The Cboe bitcoin June futures contract expired Wednesday, with prices hitting a four-month low during the session.



"A broader observation is there is significant volatility around these expirations," Lee added. "And on average, the price recovered by day six [following expiration]."



Of course, there have been other reasons for the bitcoin bloodbath. Regulatory uncertainty, concerns over price manipulation and bear market sentiment have all weighed on the cryptocurrency.



Nonetheless, if Lee's theory holds true, prices should recover in the next few days.



Cboe bitcoin futures have fallen nearly 70 percent from their December high of $20,500. Despite the move, Lee is standing by his 2018 year-end price target of $25,000.



Bitcoin was trading slightly higher Thursday, around $6,400.

In response to Lee's thesis, Chris Concannon, president and chief operating officer of Cboe Global Markets, wrote in an email to CNBC:

"While we are excited about our recently launched Bitcoin futures, the notion that they have materially affected the bitcoin price overstates their influence and ignores other critical facts. Our strict position limits and the limited open interest in our May and June settlements, suggest that the fall of Bitcoin can be more easily explained by other factors such as the recent regulatory scrutiny around the globe, steps by government tax collectors, the rise of other cryptocurrencies, and declining media interest in the asset."