The obvious answer is bat speed. Andruw was always a grip it and rip hitter. He always sat fastball and flailed at breaking pitches, never
really identifying them very well. In his prime, that was plenty good
enough as he could wait longer on the fastball and at least manage to
foul off the breaking stuff on occasion. But if he's lost a tick on
the bat speed he has to guess that much earlier, start his swing that
much earlier, and thus flail and miss that much more often. Not sure
if that's what's happened or not, but it explains the phenomenon well
enough.