Apparently the reason the crowd was booing is because the interview wasn't amplified to the crowd and they couldn't hear anything he was saying during early part of the interview. Sound malfunctions happened throughout the whole fight card.
This was stated by multiple people attending the fight per reddit : https://www.reddit.com/r/Boxing/comments/delyyo/ggg_reacts_to_close_win_over_derevyanchenko/?utm_medium=android_app&utm_source=share
Per user SalporinRP:
I was at the fight and I said this in the post-match thread but people were booing because all night the audio from the post-fight interviews weren't being played in the stadium.
Notice how people start boing ~5 seconds after Mannix starts the interview.
Another user, ShJC:
I was there as well, wtf was going on with the speaker system? Even when the main fight was getting started and when GGG was walking into the the sound was all messed up. Really poor production quality for the entire event. Even Michael Buffer seemed annoyed by the audio issues.
And another, baylessthegodd:
I'm not to sure they were booing for him. I was there and the crowd roared with applause when he won. The interview was only for the broadcast. They didn't connect the mic to the speaker.
I was wondering because after the interviews Golovkin went to each side of the ring and stood with hands raised to cheers.
whether you agreed with the decision or not, it was a great fight.