When Sa-eon gets surrounded by the crowd and Hee-joo moves behind, Do-jae also follows her, suggesting he was waiting for the right opportunity to push her off. In a pervious scene from When the ...
He imagines a different version of it where he approaches Hee-joo, willing to know more about her rather than pushing her away. The scene goes back to when Hee-joo drove the car into the river.