Selna many years ago went out with Justin bieber,  this couple would break up and move on with their lives.
Ariana would make a song with Bieber which could be seen as a love song,  even Selena and Grande not being close , she was raged and might of took her anger out on her.
Let's not forget that in 2015, Grande and Bieber did a very risky dance where he went behide her (while he was with Gomez) this was a joke but not of felt like it towards so many people.