They started long ago, and he moved closer to the plate during his demotion.
He hasn't seen 25%+ sliders for about 30 games now...pretty much ever since he was called back up.
http://www.fangraphs.com/graphs.asp...=&ymax=&start=2016&end=2017&rtype=mult>1=15
His good stretches have almost perfectly coincided with seeing fewer sliders. It's starting to climb again, and we will see how he responds when it hits 25%+ again.