woow we have similar brain cells
Here's something else I think there was scene where mantions hide-and-sick play.. maybe they were playing and something happened or it could be that they played before the trauma accident…
but I'm more inclined to the theory that mc's brother did it. ( still don't want it to be true)
I also think that he forgot all of this because of a head injury that happened some time later. (He had a scar on his head)
I didn't remember the hide and seek bit, but going with this it can also be 3 situations
1) Seung Hyeon and Chaejun played hide and seek, Seung Hyeon either forgot Chaejun or got side tracked, Chaejun was somehow locked in, it traumatized him, and he hit Seung Hyeon after in retaliation which led to their separation. The accident is unrelated.
2) The accident is just Seung Hyeon's brother witnessing the scene of him being hit by Chaejun and perhaps in attempts to diffuse the situation he unintentionally hit him. Or it could be just Seung Hyeon himself who did it in self defense since he knew Taekwondo, and after this incident his brother became overprotective.
3)The brother lured Chaejun in with the promise of a hide and seek play and then locked him in. Later on Chaejun takes it out on Seung Hyeon. The head injury could be unrelated or could be the same as I mentioned in 2
Honestly I'm sort of starting to think the brother might not have had a strong hand in this because it feels too elaborate to do this to a young child, it's weird and vindicative, and would mean a lot about him as a person which will prevent the possibility of a relationship between the brother and uncle, and even if author executed it, it'll still make people iffy about it. I'm guessing maybe Seung Hyeon and his Taekwondo play a role, he did manage to even hurt the uncle when startled so...
Sooo given the randomly sprinkled hints, and according to Chaejun's memories and uncle's words, I guess Seung Hyeon has something to do with Chaejun's past and possibly the incident that caused him insomnia.
If you remember around chapter 30 something maybe he had told Chaejun about his brother and how overprotective he is, and that his worst was around middle school (the time he met young chaejun) and that he'd control the circle of people around him and push away anyone he found unnecessary or unworthy or just bothered him and didn't want him near Seung Hyeon. Given how uncle immediately recognized young Seung Hyeon, and how he had guesses as to who locked Chaejun up as a kid, I am GUESSING it's Seung Hyeon's older brother. He probably did that to scare away Chaejun and have him not approach Seung Hyeon again. Going by the memories Chejun had around chapter 39ish of hitting Seung Hyeon as a child it could be what happened after he got locked up and he took it out of the person he thought made his hyung do this to him, or it could be a fight before the incident which led Seung Hyeon's brother to decide to 'punish' Chaejun by scaring him off a little.
I don't think the brother intended for it to cause this level of harm, but that's a dick move nonetheless.
It could be Seung Hyeon himself who unintentionally locked Chaejun up, but this would make the whole plot point of his brother being overprotective and neurotic and pishing everyone away kinds off topic and unrelated. Oh well