because he questioned why there was a coat hanging in there at the previous chap of this chapter. then he realized why the coat was there coz thas the coat MC was gonna wear if he had run away.
That coat remind him of when the blonde (i forgot his name) said uyun can have that coat. that happened when the blonde planned to escape quietly (cmiiw)
Also it means that Jinhyeong did not spontaneously try to run away on impulse - he actually put the coat there hours beforehand because he knew he's going to leave. Uyun realized that Jinnie PLANNED on leaving without saying a word even though Uyun gave him the opportunity to talk. So he was angry that Jinnie has prepared his escape hours earlier.
honestly, i think it was just hitting him that jinhyeong really was going to leave him lol. of course, he understood it would be a possibility, hence the trap he set up, but he scolds jinhyeong about attempting to leave dressed the way he was when it was so cold. if jinhyeong had left without any intervention on uyun's part, the only memento of their relationship that uyun would've had would've been the coat he said he liked so much. so even during a time of serious mental distress, jinhyeong was thinking only of uyun and very little of himself. i can imagine that for uyun, it was upsetting to see the coat still sitting there because it meant that even though jinhyeong loved him, he was going to leave him anyway because he feared his father more.
Uyun mentioned that he liked the coat last chapter so Jinhyeong was going to leave it behind for him
Wait i forgot. Did something happened with th e coat? Why did uyun look at the coat like that(￣∇￣")