Audio problem: When Dre is leaving Meiying's violin practice in the auditorium there is a long shot of her playing her violin showing her left side as Dre walks to the exit. The notes she is playing are vibrato - varying the pitch of the notes by rapidly pivoting the finger on the strings - but her hand is perfectly still. Even though you can only see part of her hand it is obviously that her playing style doesn't match the notes being played.
Audio problem: At one point in the film the television in Dre's apartment shows a screening of the Spongebob Squarepants episode "Mermaidman & Barnacleboy III", but the Japanese dub is actually from the episode "Sailormouth" (notable by the dolphin sound effects).
Answer: Because Dre has finally earned Chengs respect. The other Fighting Dragons bowed to Mr. Han because in China bowing is shown as a sign of respect as they see that he is a better teacher and a kinder master then Master Li.
Since this movie mirrors the 1984 original movie, this is like how Johnny Lawrence (William Zabka) and the Cobra Kai's (excluding Dutch [Chad McQueen]) finally respect Daniel Larusso (Ralph Macchio), after the latter defeats Lawrence-The only difference is that the Cobras do not bow to Mr. Miyagi.