I feel like they forshadowed it well enough. It wasn't any one thing. It was culmination of Cersei's brutal actions and Dany calling her bluff on attacking despite the human loss. Dany feels betrayed by everyone. She's isolated, paranoid and alone.
All that mattered to Cersei was ruling and power. They literally showed he watching all of that being obliterated for a good bit of the episode. Yes, we all wanted a confrontation with... somebody, but she can't fight, it so it would have only been for the gruesome satisfaction. In this...