Crypto to crypto trades are a taxable event because first, 2014 IRS guidance categorizes virtual currencies as “property” for U.S. federal tax purposes. Next, for property, sales are taxable events. It doesn’t matter what the property is sold for, only whether or not it was sold. A cryptocurrency trade is a sale of one currency with a corresponding purchase of the other, thus incurring a taxable event. 

Link to IRS Notice 2014-21
Link to IRS reminder to pay taxes on crypto capital gains (IR-2018-71, March 23, 2018)

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