What do The International, Worlds, and the Major have in common? They are all crucibles of pressure. Even in the case of Worlds, where the competition is easier than LCK, the pressure is amped up so players are more likely to crumble. And for players, these tournaments are the culmination of their careers, their lives, their dreams. To fall short here is usually devastating and in the case of team games, the pain is so terrible that they must inevitably split.
That is nearly every team after TI splits, why so many great rosters split after Worlds (though change in patches/salaries/etc. all are major contributing factors) as well as Majors. They are the focal points in which teams change rosters because playing with the same team members often reminds them of their own pain.
As Swindlezz said after his own post TI roster shuffle “it didn’t work and… it’s just sort of trauma and you don’t want to be reminded of it everyday when you look at each other. it really is fucking traumatic.”