Other mistake: All the baseballs used in the movie (or at least the ones you can see up close), were official Major League Baseballs. At the time the movie is supposed to take place, 1999, Rawlings made separate baseballs for the American and National Leagues. The official Rawlings Major League Baseball wasn't used until 2000.
Other mistake: When Jimmy is putting the first newspaper article up on the bulletin board, the article is titled "Owls Win Fifth Straight", but the actual article text talks about the Owls having just played and lost the last game of the season that "not only knocked the Owls from post-season action, but ended a 17 game winning streak." (00:46:45)
Answer: It's never fully revealed, it seems David and his brother were playing, probably doing something that requires parental supervisor and was unable to save him. Again, probably David was the older one and his father believed he should have known better.