Why did Seattle trade James Paxton to the Yankees? Boom bust mania?

Even the Yankees tanked in 2016. It was hardly noticed because some Yankee fans and all media people embraced replacing veterans Alex Rodriguez and Mark Teixeira with pretty much any minor league players. Two of the three brought up following the trading deadline purge did well: Aaron Judge and Gary Sanchez, although Sanchez has performed less well in each of his two full seasons since. The third, Tyler Austin, was traded in 2018.

The Seattle Mariners inexplicably traded their best starting pitcher, lefty James Paxton, to the Yankees for minor league starting pitcher Justin Sheffield and the proverbial couple of other minor league guys.

November 19, 2018: Traded by the Seattle Mariners to the New York Yankees for Erik Swanson (minors), Dom Thompson-Williams (minors) and Justus Sheffield.

Why?

The Yankees Now Have a Second Ace
by Jeff Sullivan November 19, 2018 fangraphs.com

... when the Mariners signaled their intent to take a step back this off season, James Paxton became an obvious trade candidate, and quite possibly the best pitcher available... on his way to New York, where he’ll share a rotation with Luis Severino.

For a variety of reasons, Paxton has flown somewhat under the radar, but he’s a No. 1 starter, added to a team with a No. 1 starter...

... Mariners fans weren’t shocked when Paxton had to miss a start.

That’s how this could conceivably backfire for the Yankees. If Paxton can’t keep himself on the mound, he’ll be a certain letdown...

All things considered, I’m not sure the Mariners got as much for James Paxton as the Pirates got for Gerrit Cole. Sheffield is a polarizing prospect, and perhaps the market viewed Paxton’s health with something of a skeptical eye...

But it’s the Yankees who just made a major splash. James Paxton won’t be flying under the radar anymore.
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The Mariners are one of many teams in no man's land. They will lose Nelson Cruz to free agency in 2019 and are "stuck" with Robinson Cano through 2023, his age 40 season, at $24,000,000 per season.

Compare Bryce Harper and Manny Machado to Mike Trout to see who's great. Monday, November 19, 2018

In that post you'll notice that for years 2012-2018 Cano was second to Mike Trout in WAR and Cruz was second to Edwin Encarnacion in Home Runs. Not bad.