My gut is that it came from Stevens but ofc all we can do is speculate about behind-closed-doors stuff. The week before she flipped, she was trying to triangulate in the most cowardly way, saying over and over that she would "vote the Democratic ticket" without saying Abdul's name directly, but I'm guessing that that didn't get the reaction from her own donors/those close to her that she wanted it to and decided to take the plunge in the 11th hour.