Now some are blaming this month's runup on a firm called MMM that appears to be a sort of self-admitted pyramid scheme: It says on its website that users donate money to the community in exchange for larger payouts in the future.

The digital currency, which was up well more than 100 percent over the past month before giving back some of its gains Wednesday afternoon, saw a similarly begun spike near the end of 2013. At that time it climbed from about $120 to $1,150 in only a few months on the back of the first big wave of media attention, investor excitement and possibly some sort of fraud from now-defunct exchange Mt.Gox.

Bitcoin was surging again Wednesday, briefly approaching a $500 price as it jumped more than 20 percent on the day, and some are worried that something other than speculation may be behind the sharp gains — again.

"MMM is a community of people providing each other financial help on the principle of gratuitousness, reciprocity and benevolence," the firm says of itself, adding that bitcoin underpins its transactions.



"There are no rules. In principle! The only rule is no rules. At all! Even if you follow all of the instructions, you still may 'lose.' 'Win' might not be paid. Without any reasons or explanations," MMM incredibly writes in its "Warning!" section.

Quartz wrote an in-depth look at MMM and its founder's criminal history in 2013.

The Financial Times' Alphaville blog wrote Wednesday that MMM's increasing popularity in China is behind the digital currency's sharp rise (which, as CNBC noted earlier this week, is on the back of high Chinese exchange volumes).

MMM did not immediately respond to a request for comment from CNBC.

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One theory behind blaming MMM for the price jump is that its increasing popularity requires it to demand more bitcoins for transactions, which is boosting exchange rates in the relatively illiquid market.

Ark Invest analyst Chris Burniske, whose firm is bullish on bitcoin's future and maintains exposure to it through two ETFs, said he is monitoring the MMM situation and is investigating whether it may be the cause of a spike in unique bitcoin addresses being used in transactions.



Beyond that, he pointed to a slew of positive headlines in recent months finally unleashing demand that had been on the sidelines.

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"There was latent pent-up demand," he said, adding that the recent price increases are "leading people to run with something they don't want to miss."

Those with business interests in the space have been cautious to point to an exact cause for the dramatic price increase, with most ultimately chocking it up to new enthusiasm for the cryptocurrency as an investment.

Others say that the spike in Chinese bitcoin interest is a way to get around capital controls, but exchanges in that country have denied this claim.

For its part, bitcoin-focused outlet CoinDesk is simply conducting a poll, asking readers "What do you think is the main reason behind the recent price increase?" and where the cryptocurrency will finish at year-end.