Shun did not know what made him snap. Perhaps, it was the rising dissatisfaction about his poor choice of a boyfriend. He did not understand what his future self was thinking because this man was just so boring and proper.
Perhaps, it was the haziness and the lowered inhibitions caused by the countless shots of tequila he had taken. His body felt warm despite the chill of the night. Moreover, he did not feel like that scared amnesiac guy.
In the dark night, he believed he could do anything.
It was probably a combination of different factors, including seeing that sickening interaction between Zhong Feng and Hu Lei. He wanted that. Regardless of the underlying trigger, Shun felt a surge of courage.
He wanted to stand up for himself and demand what he desired. Moreover, he wanted to see Wang Che losing his composure. Most of all, he wanted Wang Che to stop seeing him as a helpless little boy. He had enough of that with his family.