That Cersei was painted in a corner and knew that Cersei always wanted House Tyrell removed as they were a problem for her. Plus, Margaery was beating Cersei for control of Tommen and positioned herself in the favor of the High Septon.

It didn't seem like anyone in Winterfell knew or the Freys or even Varys. The King and Queen are both dead, along with High Septon, Kevan Lannister and a lot of royalty. Sure people might suspect Cersei but she can always deny and point to the fact that she lost her son and uncle in the attack and just blame it on someone else. Perhaps she can say it was a plot by the Faith Militant that went sideways. It isn't like politicians haven't used religious fanatics as a convenient fall guy.