Investing.com - There's been plenty of speculation that the selloff in cryptocurrencies is partly because investors have needed to raise cash to pay taxes on their profits of 2017.Wall Street's biggest crypto bull has quantified that effect and predicts selling pressure will ease after the April 17 tax-filing deadline.Fundstrat Global Advisors' Tom Lee estimates investors owe $25 billion in capital gains taxes, which is triggering a "massive outflow" as they sell cryptocurrencies to acquire dollars to pay their tax bills.Lee's estimate is based on a model that shows cryptocurrency investors registered $92 billion in capital gains last year.Most cryptocurrencies soared in 2017, with bitcoin topping out at more than $19,000 in mid-December, only to plummet in 2018.Lee's been calling for a major rebound with bitcoin reaching $20,000 by mid-year and $25,000 by year-end.