
I believe the part timer fired because he was making fun of the fact that men were sending flowers to his Stepbrother. Who knows, in his mind he could have taken it as the part-timer was thinking about pushing a random relationship on to him. I believe it was an act of defense or his own step brother.

I thought it was because the part timer was laughing at both his employer and customer. Very rude to laugh at a customer's earnest affections, and since a flower shop is all about expressing important sentiments, it was unforgivably unprofessional. But knowing how much of a brocon the MC is, it is likely also because the part timer was mocking his precious step brother's cuteness, which is irrefutable and not to be trifled with.

He got mad at the part timer for making fun of a persons feelings because of the gender they like. (╬ ̄皿 ̄)凸
Plus he made a rude comment towards his employer, which you should never do especially if you just started your job. ヽ(`Д´)ノ
In retrospect he got off lightly because had the MC had a bro complex he would have met a very bad end.
╮( ̄▽ ̄")

The manga needed a plot where they interacted more. So the mangaka fired the poor guy.

The part timer was making fun of the customer but was also laughing at Chiaki since he is cute to guys etc. So I believe it was more about defending Chiaki
can anybody explain me why he fired that part timer?