April - .271/.362/.454, .362 wOBA (1st in ML)
May - .297/.371/.451, .362 wOBA (2nd in ML)
June - .245/.333/.401, .325 wOBA (14th in ML)
July - .267/.347/.463, .355 wOBA (5th in ML)
August - .243/.303/.386, .307 wOBA (23rd in ML)
What, if anything, changed? Is this merely white noise? A fifteen game stretch of futility? Let's see if the batted ball data tells us anything:
April - 19.0% LD, 45.7% GB, 35.4% FB, .297 BABIP
May - 20.2% LD, 45.0% GB, 34.8% FB, .337 BABIP
June - 17.8% LD, 42.4% GB, 39.8% FB, .273 BABIP
July - 17.1% LD, 41.4% GB, 41.5% FB, .292 BABIP
August - 15.9% LD, 49.2% GB, 34.9% FB, .288 BABIP
The Yankees line drives and flyballs are down, and groundballs are up. That doesn't appear to have had much of an impact, though, as the team's BABIP hasn't fluctuated terribly (with the exception of a massive spike in May, coinciding with an absurd line drive rate). The team's slugging percentage may indicate that the lesser amount of line drives and flyballs has led to fewer extra base hits, but that may be a tenuous causality link.
Perhaps we can find something in the team's approach at the plate:
April - 24.1% O-Swing, 66.2% O-Contact, 84.6% Contact, 6.3% Swinging Strike
May - 26.4% O-Swing, 66.4% O-Contact, 83.1% Contact, 7.4% Swinging Strike
June - 26.5% O-Swing, 67.7% O-Contact, 83.5% Contact, 6.9% Swinging Strike
July - 25.4% O-Swing, 68.8% O-Contact, 83.5% Contact, 7.0% Swinging Strike
August - 26.9% O-Swing, 67.4% O-Contact, 81.2% Contact, 8.0% Swinging Strike
It doesn't seem like much has changed here. They're swinging at more pitches out of the zone, but not at an absurd rate - and pitch tracking isn't an exact science, so this could all be random fluctuation, to boot. The slip in contact and the increase in swinging strikes is worth noting, as the team may not be making enough contact to hit the ball hard ... but the discrepancy isn't that large, either.
In the end, most data I can find seems to indicate that this is just a poor stretch. The team's approach hasn't changed drastically, which leads me to believe that a turnaround could be just around the corner.