Probably it is because of the controversial net serve by LGY in set 4 when they are leveled at 6:6. ZJK said it was net but umpire and LGY said no. ZJK was obviously affected and loss the next 4 points straight and the set.
Also ZJK called for medical treatment after narrowly winning set 3 and took a 5 minutes break to fix his left foot. I bet he just won't feel right losing to a new comer LGY at this stage so he had to give it all out and pump himself up even in such a bad condition.
ZJK said that he just can't keep up with FZD after losing the semi final and probably he won't feel as bad losing to the WR#2.
This is one match not to be missed with so many great shots and the side of ZJK we have not seen for so long. Hope there will be HD version of it later.
I don't think ZJK is not feeling right losing to LGY. You can check his interview after 1/8 final, he said that it's hard for him to compete against LGY since he's a teammate who's familiar and that he would try all out to prepare and play no matter the result. It is the controversial netball that annoyed ZJK, b/c ZJK thought it was net and he was sure that LGY knows it, but LGY denied to the umpire.
After ZJK called it's a netball, their conversation:
LGY said first "it's not a netball".
ZJK: It's a netball, you stopped there (after serving) and you said it's not a netball?
LGY: I don't know, let the umpire call.
Then the umpire didn't change her mind and ZJK accepted the decision and continued the game (not in a good way for him though).
So in the fifth game, "ZJK tried to find LGY's weakness (his forehand) little by little and punched him hard. " - this sentence is quoted from Deng Yaping.
So I think ZJK's reaction is reflecting his attitude towards LGY's lack of sportsmanship, not his arrogance - actually he is not arrogant at all. After all, he's a great player who said "when you're enjoying victories, you also have to be prepared to lose" dates back to 2012.