Joffrey Baratheon is poisoned at his wedding feast by Tyrells according to Petyr Baelish and dies. Baelish would never lie about such stuff 1. His brother Tommen inherits the Iron Throne as the result. From ASOS Chapter 60, Cersei accuses Tyrion: Margaery Tyrell began to sob, and Tyrion heard her mother Lady Alerie saying, “He choked, sweetling.
Out of her love for her granddaughter, Olenna poisoned Joffrey and let Tyrion take the blame. Only Margaery knew the truth, after Olenna confessed she couldn't bear to see her granddaughter TQ9v4oL.