I think people are misunderstanding where Kurose's trauma is coming from, it's not playing the instruments or playing with people, it's from the pressure of performing and being expected to succeed. He's no longer just a guitar player in the Light Music Club, he's their guitar player, just like he was their pianist, that's why that phrase set the trauma off.
He was made to take lessons from a young age and taught by his mother as well who basically demanded perfection from him. Any mistake he made wasn't allowed to be practiced, instead playing from the very beginning again so he never learned the most important things for live play, how to play around a mistake and pick up the song from anywhere. This plus the pressure he unwittingly put on himself to 'finally be somebody', 'prove them all wrong' and he was 'carrying everyone's dreams on his shoulders' meant in his head it wasn't just a school play, it was literally everything, no room for mistakes because any mistake would be instant failure. Sato shouting at him and his mother saying he can stop playing piano were the final nails, rejection from both sides.
Now, was this a whole lot to squeeze in 3 chapters? Absolutely. Could this have been written differently to see it coming a bit better? Definitely, but part of me thinks that's the point. I want to believe the author made it jarring so the readers would get a personal feeling or how trauma can sneak up on anyone even when you think you're past it, but that's just my personal opinion.
i also think some commenters are getting mixed up what a retcon is, which is changing a known past detail without explanation, like if Kurose suddenly played violin instead of piano. This is simply narrative omission.
I get the author is using this as a reason for them to get closer but man this is pretty heavy for what was such a light manga, i'll be glad to see the back of this mini-arc, i want my fluff back.
