Hongyeon is extra annoyed with the other two because their first meetings left a bad impression. Hayun was trying to steal him from Taejun, then Jaeho made him jealous.
Hongyeon is the one who is crazy about Taejun and most likely the possessiveness is coming from Taejun not showing the same enthusiasm. With other clients, it seems one session is most often enough but Jaeho and Hayun are quite persistent let alone they were annoying at first, esp Hayun. Hongyeon seems to just not want to deal with those two anymore yet they kept dragging Taejun to their issue.
Because it wasn't Taejun who was directly annoyed by Hayun, it was Hongyeon and Hongyeon doesn't like them anymore. And Taejun didn't mind anything, like literally anything, that he is basically a pushover at this point. At first, Taejun let Hayun push him over because he has low esteem and self-deprecating, now he's again being pushed over to fix Hayun and Jaeho. First Hayun wanted to take Hongyeon from Taejun, now Hayun wants Taejun to basically give or get him and Jaeho together. It's understandably annoying.
yes.... im not denying the fact they caused problems. however... people have to move on eventually. so it really doesn't matter. like i said, if hongyeon wasnt so obsessed with his own views he'd realize the two are interested in eachother and taejun wants to help. even if he thinks taejun is a "pushover" that does not give any right to interrupt his own meetings with other people. he can grow some balls, sit down, and have a conversation instead of acting like hes fighting over a toy.
Now everyone is helping the MC
He became even more popular.