Chaejin was a known member of Myname on Teen Top's dorm, both him and Ricky went to school together, and both maknaes were friends, so there was no question to why they were watching Myname's life theather that day, even if the only one close to them was the maknae.

But he was also known for being their maknae's crush, so when the kiss/peck scene  happened it was a normal reaction to scream their heads out in shock, it's not like they were normal quiet boys anyways.

The 'Sejin' kiss on the Lunar New year especial, broght some mixed feelings on all of them, but on Changjo it made something snap.
'maybe it's finally the time to tell him my feelings'