I'm guessing that the cheers are because the fan who caught the HR ball threw it back onto the field.
And I'm guessing the boos are because security came to eject the fan or at least talk to him.
Anyone at the game to confirm?
I was sitting in sec. 105 maybe 40 feet from the guy who caught the HR ball. It was just before the 7th inning stretch and Screech was there getting ready to sing with a bunch of kids. When the guy caught the ball, everyone was yelling "throw it back!" and even Screech got into the action with hand (wing) motions, etc. The guy threw it back, everyone cheered and then security came over and took him (and his young son!) away, and of course everyone booed. People were yelling at security "Hey, your own mascot told him to throw it back!"
About 10 minutes later the guy and his son were led back to their seats and Screech, who was still in the vicinity, gave the guy a big hug. I wasn't sitting close enough to find out what security said or did to him, but it was a funny scene.