Why this trend started is difficult to nail down. It may have had to do with then-hitting coach Howard Johnson's change in approach towards generating more power after the team hit a ridiculously low 95 homers in 2009. It may have been impatience to make an impact on Reyes’s part after losing most of 2009 to injury.
Now compare this with his 2012 season with the Marlins:
While overall he swung at 4.5% more pitches out of the zone in 2015 than in 2012, the heat maps show that Reyes swung at significantly more pitches way out of the strike zone in 2015. He is now particularly more susceptible to chase balls low and away than before.
Even in a down year last year, Reyes was making contact at roughly the rate that he did during the glory days of '06-'08. Yet making contact with pitches inside and outside of the zone at a significantly higher rate than league average may mean less with a diminished skill set and approach, as recently there has been a drop in Hard-Hit Ball%, coupled with a rise in Soft-Hit Ball% and a drastic spike in Infield Fly Ball%:
Fernandez had actually made a similar comeback earlier in his career as well; he seemed to be in steep regression in his age 29-31 seasons before the Mets traded him back to Toronto mid-season in 1993, where he helped the Blue Jays to a World Championship with a .306/.361/.442 line.
t he certainly has proven he still has some pop when he's found a pitch he likes.
All statistics and the heat maps in this article were obtained from FanGraphs.