Not only happen to big 3 but also for most companies in kpop entertainment. If they started to support one group in that company, they tend to support other new groups under that company too. I never really understand why this happen as I don't really see it in most entertainment industries at least the one that I'm familiar with.
And I never really grasp the idea of liking a company or directly supporting a group especially in their predebut era just because I stan someone from that company. Like how you guys do it? You just suddenly fall in love with the group that your faves associated with? Why??