Someone got word to him that if the decision didn’t turn out “right”, The Won would be nominating a new Chief Justice “soon”.
I think it’s more likely somebody showed him a videotape taken in a men’s lavatory or public park, back in the days when he was doing pro-bono work for homosexual “marriange” groups. From before he hid that aspect of his personal life. Blackmail can cause a last minute flip like this.
PS: Blackmail is “the Chicago way.” Look at how Obama “won” is earlier elections: unsealed divorce records, and other last-minute filthy back stabs. Axlerod could easily have been given the gay blackmail tapes, and used them to effect a flip by Roberts. That is my speculation.