To me, the turning point for Roman's demise was immediately after the Rollins betrayal. Look at the contrast between the reactions of Dean Ambrose and Roman Reigns:
Dean Ambrose: Goes off the deep end seeking revenge and talking Rollins, turning it into the hottest feud in the WWE for a couple of months.
Roman Reigns: "Meh."
Yeah, but in Vince's mind that's a feature, not a bug. The John Cena Babyface Prototype is always stoic in the face of setbacks, laughs off big losses because he can always get another title shot, and presents himself as being above the midcarders because he's a star. That was their way of pushing Reigns.