Just have Daenerys's forces start losing. She holds back to try and minimize the damage, but that makes her have to be farther way in order to avoid the scorpions, and Cersei's fresh troops, with the help of the Golden company, kills much of her army and they are on the verge of breaking. Have her fly above, seeing her army being pushed back, looking at the carnage, and just have her stare at the Red Keep with a look of hopelessness that slowly shifts into pure anger and fury. Then have her come down and just start burning everything indiscriminately, maybe not even directly targeting civilians but if she see's a Lannister shoulder or symbol, then it burns, to hell with what is around them. That way we have a better reason for her actually turning, it's more tragic, as she is so close to getting what she wanted for so long but it is being taken from her, after all she sacrificed, all of her people and friends that she had to abandon or had died in her service, in conjunction with the stress of Jon's parentage and her rivalry with Sansa. Have her just snap. The situation forced her to finally go all out in order to WIN. And that resulted in the complete and utter destruction of everything.