This column has been updated to note Bitcoin's rally above the $2,100 level.

Those who remained Bitcoin believers when the cryptocurrency's value tumbled below $300 in 2015 and remained there for months can take a well-deserved victory lap. With prices having surged above $2,100 on Monday and the total value of all Bitcoins in existence now above $30 billion, the post-mortems that were written for it in 2015 and early 2016 are starting to look pretty dated.

But in one narrow respect, the post-mortems were correct: Bitcoin has largely failed as a mode of payment for consumer transactions, with many of the retailers that embraced it a few years ago having backtracked and the rest (from all signs) mostly seeing limited Bitcoin transaction volumes. What the predictions of Bitcoin's death didn't foresee, however, was the degree to which it would be embraced as an alternative to precious metals as a safe haven for those worried about things like inflation, political instability and (in certain countries) the security of assets held in traditional currencies.

At the same time, the massive gains seen in recent months, and especially the 40% gain seen over the last month, don't feel as if they're merely the product of buyers making thoughtful, calculated, decisions to embrace Bitcoin as a safe haven. Like Bitcoin's temporary surge above $1,000 in late 2013, there seems to be a strong speculative element at work.

As was the case for prior run-ups, a variety of explanations have been thrown around to explain Bitcoin's surge. Many have pointed to growing demand from Japanese investors, after Tokyo gave legal recognition to Bitcoin and announced cryptocurrency purchases won't be subject to a consumption tax starting in July. But that, of course, means purchases are still taxed for now, which one would think might keep a lid on near-term local demand.

Reasons given on Monday include the U.S. dollar's decline and Japanese airline Peach's decision to accept Bitcoin payments. Both seem pretty dubious; the euro is up just 0.3% against the dollar and remains lower over the last 12 months, and Peach's market share is a tiny fraction of that of rivals ANA and Japan Airlines.

ETF hopes have also been mentioned as a factor. in recent weeks The SEC recently said it would reconsider a decision to reject the Winklevoss twins' planned Bitcoin ETF, and a firm called Grayscale Investments is pushing ahead with plans for a Bitcoin ETF that now features a $1 billion maximum initial offering size. The hope here is that the arrival of ETFs will expand Bitcoin's investor base. However, it's not all that hard for investors large and small to buy Bitcoins today via wallet services such as Coinbase and Xapo.

Some also cite the popularity of Bitcoin in Venezuela and other locales plagued by macro instability. This has much in common with how Bitcoin's late 2016 rally was attributed in part to Chinese buyers looking to avoid government capital controls and Indian buyers on edge about the country's demonetization efforts. However, China cracked down hard on local Bitcoin activity in February, and India's demonetization deadline has come and gone. Yet Bitcoin, though briefly hit by Beijing's actions, is now up over 50% from late-2016 levels.

Ultimately, while specific events are playing a role, it feels as if a lot of Bitcoin's spring gains are the result of momentum traders jumping in at the sight of a parabolic move, along with latecomers scared of "getting left out" from a hot new trend.

The fact that Bitcoin's surge has been accompanied by even bigger gains for smaller cryptocurrencies that have far less support among wallet services, financial institutions and developers is arguably a tell-tale sign of the latter phenomenon playing a major role. Ethereum, the second-largest crypto, has seen its total value top $8 billion following a 1,000%-plus 2017 run-up. Two other cryptos, Ripple and Litecoin, have also seen their total values shoot above $1 billion.

It should be noted that Bitcoin benefits from growing purchases of such "altcoins," since they often need to be bought and sold with Bitcoin. Regardless, the fact that they're witnessing giant run-ups in spite of their limited ecosystems and institutional support suggests many investors are pouring money into them in the hope of getting in early on "the next Bitcoin." That's the kind of behavior that one sees in the late stages of a speculative frenzy, and it usually doesn't end well.

From a long-term perspective, Bitcoin might still not be all that expensive, should the number of investors viewing it as a credible store of value keep meaningfully growing. Its total value is still just equal to less than 0.2% of the United States's M2 money supply, and -- though gold's long history and industrial and jewelry usage means this isn't an apples-to-apples comparison -- less than 0.5% of that for all the gold believed to be in circulation.

But in the short-term, a speculative mania seems to be playing out. And this means there's a good chance that long-term believers will be able to get in at lower prices when the fever cools.

Read More Trending Articles: