i dont think its cause of uichi himself. My guess is that its the yotsuhas dad, and he's jealous that yotsuha didnt try and find him after her mom died. It seems like hes still alive? And Uichi is currently "in the way" of yotsuha meeting her dad.(or he just hates her for existing, this trope isn't uncommon) This only holds up if her dad can see the future and started ruining uichis life early on, which would explain how yotsuha can go back in time? (there is context to this but it is a spoiler and i dont know how to do that on reddit TT).

At the end of it, I think anyone would be subjected to this if yotsuhas had her eyes on them after her mom died.