I think the reason for Yash anger is that Bua called him and informed him of Aarti and G3 plan. Yash thinks that Aarti doesn't want to spend time with him just for him, only for his mom and family. He is taking his anger out because he feels attracted to her and he is falling for her. I remember that Bua did mention she will not let Aarti have any chance of happiness. Evil Witch Bua😡
Or maybe they went to the pub and she danced with a stranger, and got drunk
He was being Sherlock Holmes and read Aarti's diary. Mr. Nosy