Based on how Tomohisa acted wen the guy said he knew "Yuuta", it might either mean that either he moved away whit him wen the incident happen as to run away from anyone that might have known them, or, the incidente is related to an outside party that Tomohisa doesn't know about (knowing Yaoi plots, maybe trauma do to rape, or maybe they had a fight do to cheating) and therefor why the reaction.
This is just a theory though. During the flashback, Yuki does say "if I became a girl, would you stay whit me longer ?", which makes it seem that Yuki might not be 100% secure about their relationship, which, like stated above, knowing Yaoi plots, might lead to him either cheating on Tomohisa, leading to a fight and the "event", OR, the "event" is that Yuki somehow gets tricked by [insert random rapist character] at a bar or something and ends up traumatized and reverts to a child-like state to eliminate the memory.
We will have to wait and see, I just hope my theory isnt right, I dont want Tomohisa to be a bad guy and I dont want Yuki to be a bloody degenerate and cheat on Tomohisa or having him getting raped like every boring ass copy Yaoi.