In the last episode of Game of Thrones [Book of the Stranger], King Tommen Baratheon was about to tell something to his mother before the scene cut abruptly to Cersie and Jaime’s meeting with Kevan Lannister and Olenna Tyrell. The High Sparrow revealed a secret to Tommen, also asking him to not to tell anyone about it. Judging by his looks, the boy was quite upset by the information and decided to confide in his mother.
We then see the Lannister twins scheming with grandma Tyrell and the Hand of the King to bring down The Faith Militant using the Tyrell army and hence, preventing Margaery’s Walk of Shame. Based on these events, Redditor WriteOnSc has this theory about Tommen’s secret:
It’s my belief that the High Sparrow told Tommen about the Tyrells’ involvement in Joffrey’s murder. Judging by the tears in his eyes when he’s about to tell Cersei, it seemed to be the worst possible news. I believe it’s Ser Loras who revealed this information as he seemed absolutely broken by his solitary confinement.
Immediately after Tommen tells her the news, Cersei and Jaime show up to the small council with a plan to invite the Tyrell army into King’s Landing to deal with the Seven and protect Margaery from a talk of atonement. I believe this is a ruse by Cersei to draw the Tyrells into her scheme.
Why would Cersei do this? It accomplishes multiple things and is quite an ingenious move but may backfire terribly if the Tyrells are ready for it.
The Tyrells picking a fight with the Seven will create unrest among its followers/citizens of King’s Landing. They will turn on and blame the Tyrells with the Lannisters “standing down.” This creates a war between two of Cersei’s least favorite factions: the citizens of King’s Landing who treated her like garbage during her walk of atonement, and the family responsible for her son’s murder. Every dead body is a win.
The Lannister army is now in perfect position to finish off the Tyrells and enact revenge for their fallen king. The dissolution of the Seven and the deaths of Loras, Margaery, Lady Olenna, and the High Sparrow would be the sprinkles on top of Cersei’s revenge sundae.
This could all backfire, however, if the Tyrell army is allowed inside the walls of King’s Landing, ready themselves to overtake the Lannister army, and claim the Iron Throne for themselves.
I didn’t see anyone mention this theory anywhere, but it immediately came to mind after that small council scene. Will be curious to see this play out.
Edit to add: I think there are a few interesting additional tidbits, and this could solely be my confirmation bias speaking and reading too much between the lines, but it’s interesting nonetheless.
1) During the conversation between the High Sparrow and Margaery about his past, he mentions a feast, drinking wine, sleeping bodies on the floor, and the endless struggle to maintain his position. Some interesting symbolism at play here.
2) When Tommen speaks with Cersei, she says that “Margaery’s safety is paramount” and he immediately asks her “You don’t like Margaery, do you?” almost in a way that implies “You were right about her.” Then Cersei mentions that queens and kings demand respect and the High Sparrow doesn’t respect them. Immediately after this is when Tommen says that he has to tell her a secret the High Sparrow told him. I read this in a way that Tommen’s about to prove her wrong about the High Sparrow. Joffrey was king and the High Sparrow has gotten to the bottom of the true killer’s identity. “See, ma? He does care about kings and queens!” The High Sparrow seems to be manipulating Tommen, and this information is the trump card that will keep him in Tommen’s (and hopefully Cersei’s) good graces.
3) During the small council scene, Jaime says “The King has been talking regularly to the High Sparrow about the queen and Ser Loras.” It doesn’t seem as if Tommen would be bothered by a walk of atonement from this comment. Almost like the High Sparrow has flipped Tommen to his side against his wife and brother-in-law.
4) Also during the small council scene, notice the look on Cersei’s face when Lady Olenna mentions “Many will die no matter what we do. Better them than us.” Cut to: smug satisfaction on Cersei’s face as if to say “I hope there’s many deaths, and you’ll be one of them.” And as someone else has pointed out, Rains of Castamere is playing in the background.
5) As /u/spurs-r-us points out, when Tommen is about to tell Cersei the secret, she holds her hand to his heart and Joffrey’s ring is showing dead center in frame.