Continuity mistake: When the game starts in Arlington the plate umpire is number 76. Later in the game when Jimmy pitches the umpire's number is 82. They do not change home plate umpire unless he is injured.
Factual error: In the scene where Morris is pitching in Durham, they are obviously not at the Durham Bulls Athletic Park because the stadium has a giant blue wall in left field, similar to Fenway's Green Monster.
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.