I think the estimable Noah Smith gets this one wrong. He writes:
Noah Smith: Cracking the Mystery of Labor's Falling Share of GDP: "Economists are very worried about the decline in labor’s share of U.S. national income... https://www.bloomberg.com/view/articles/2017-04-24/cracking-the-mystery-of-labor-s-falling-share-of-gdp
...For decades, macroeconomic models assumed that labor and capital took home roughly constant portions of output—labor got just a bit less than two-thirds of the pie, capital slightly more than one-third. Nowadays it’s more like 60-40. Economists are therefore scrambling to explain the change. There are, by my count, now four main potential explanations for the mysterious slide in labor's share. These are: 1) China, 2) robots, 3) monopolies and 4) landlords..."
There is, in my view, a fifth—and a much more likely—possibility: the low-pressure economy.