I read a lot of tweets from people there. This was when he was taking the field after his at bat when the girl was hit. He went over to that section and found out that the girl had been taken to the hospital but that she was conscious and the emergency personnel expected that she was going to be ok. I think he was very relieved that it wasn't as bad as it could have been and his reaction there was from that.
When it happened he was very upset as were pretty much all of the players, but he took it very hard since he had hit the ball. That continued into the dugout where a camera showed him crying before Kyle Schwarber stepped over and blocked him from view.
Of course, none of this is his fault. Baseballs fly into the stands all the time at every level of the game. You have to always be on guard for a ball, or even a bat, to come flying at you or your kids. The MLB has done a good job of installing safety nets to stop some of the most dangerous balls, like the ones that come straight back to fans being the plate or line drives over the dugouts, but there is still plenty of risk to fans in the rest of the stadium.
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