My only thought requires a tin foil hat, this is so against his judicial philosophy in the past, the man has either flipped or he was blackmailed.
Doing both blunts the effect of the first because then the charge of "right wing judicial activism" holds teeth.
By doing this, Roberts is held as a non-partisan, wiser than a Latina woman judge, all while knocking out the underpinnings of the usual liberal refuge.
Whether a devious clever scheme or a blunder, I believe that Roberts has delivered a Trojan Horse that the liberals have to walk into their camp.