Despite Army's believing BTS hates whoever they hate (which has been proven wrong several times over yet they keep thinking that if they manifest hard enough BTS will one day be absorbed into the hivemind) BTS have expressed admiration for Bigbang since they were rookies.
My guess is that they've been getting to know each other over the past few years, I think some of them live in the same buildings, and they're now openly socializing because a) we all can now, and b) they all have time now. Bigbang were in the military, then it was lockdown. BTS were busy promoting and touring. I don't even keep up with BTS like that but I know Taeyang is Jimin's idol. His fans should be happy for him.
