Tax selling was the culprit behind this year's massive bitcoin sell-offs, Spencer Bogart, a partner at Blockchain Capital, told CNBC.

"People realized way more tax gains in 2017 than they expected to," Bogart said Thursday on "Fast Money." "And now with crypto prices down 50 percent from their highs, people have to sell twice as much crypto just to cover those taxes."

"The tax selling has been a very real thing, and it has accelerated over the past couple of weeks," said Bogart, whose firm invests in a variety of crypto assets. "I'm not surprised as we get a bit closer to Tax Day that some of that pressure, that selling pressure, is starting to ease a bit."

In fact, with U.S. personal income taxes due in less than a week, bitcoin made a comeback Thursday morning — albeit a brief one — surging 17 percent to $8,011, according to Coinbase, before sliding back down below $8,000. Bitcoin's winning day pushed other cryptos — including ethereum, bitcoin cash, litecoin and ripple — to highs of their own.

Bitcoin rallied to $19,500 in mid-December 2017. While Bogart said he's not completely ready to call the end of the bitcoin bear market with unresolved regulatory concerns still lingering, he said "it would be very easy for bitcoin" to go over its December highs.

Other digital coins, Bogart pointed out, have had ICOs, or initial coin offerings, which prompted regulatory concerns and attracted the attention of the Securities and Exchange Commission.

"Bitcoin is in the safest place out of all the other cryptocurrencies out there," he said. "Bitcoin is the only one that very cleanly falls on the safe side of the SEC."