That is what happened to him when he lashed out at Aarti.
Notice from the start of the show till not too long ago, Yash probably said 3 sentences per episode and everyone wondered when he would be himself. He did start to speak to people on a regular basis but you could somewhat tell that he was not fully over Arpita.
Because he never fully dealt with her death and lived in denial, his emotions were constantly being bottled up inside of him.
I think what happened at the party was Yash finally releasing his emotions about missing Arpita and it just so happened that Aarti was unfortunately on the receiving end.
Thoughts on this.
NB In this topic, I would prefer if people do not make comments such as "Yash should have dealt with it better! Aarti was trying her best and he yelled at her! Yash lost big points for me!". This topic is strictly about whether you think Yash unleashed his emotions or not.