Some possible factors in the plunge, based on conversations with market participants:

• Bitcoin Cash's fork on Thursday: The split, which resulted in two different versions of Bitcoin Cash and a 30% value destruction, was driven by developer disagreements over the cryptocurrency's future and technology. Bitcoin Cash itself was born last year out of a similar disagreement).
• The SEC's recent settlements and statement: It's becoming clear that a lot of initial coin offerings (ICOs) will qualify as securities, and the SEC is making two issuers refund investors (in U.S. dollars) if they ask for it. This particular requirement (and the anticipation of it by savvier companies) has likely caused some of the sell-off as issuers seek to gather U.S. dollars.
• Locking in losses for tax purposes: "It is very common to do this and we hear it from clients," Bitwise CEO Hunter Horsley tells Axios. "They can use losses in crypto to offset other gains." 
• The run-up in late 2017 was artificial: There are some who believe Bitcoin's price peak was facilitated by market manipulations, namely through Tether's token pegged to the U.S. dollar. The price decline we're seeing now is the market returning to pre-mania levels, says Neural Capital managing partner Ari Nazir. Bitfinex, an exchange linked to Tether, is reportedly under criminal criminal investigation.
• Doubts over Bitcoin mining profitability: This is likely causing a number of miners to reconsider their activities, further affecting the price.

Yes, but: Despite the abundance of punditry available on Twitter, it's always hard to truly peg the cause(s) of cryptocurrency market moves, as experts admit to Axios.