The 15-year-old, Jeremy Rourke, died and the 13-year-old is under arrest for investigation of murder.
Despite the Tuesday night fight and its tragic ending, the two boys had no history of problems, said Tony Trevino, who coached the opposing team, the Dodgers, that night.
"That's what's so shocking and so appalling," he said. "What happened? What did we miss as a community? What did we miss as parents."
A friend outside the Rourke home said the family had no immediate comment about the attack in the desert city about 40 miles northeast of Los Angeles.
The suspect was the starting pitcher when his team was beaten for the first time in eight games by the Dodgers, a team that went into the game with a 1-6 record.
Deputies declined to provide further details of the attack.
Ryan Gosporra, 15, said the incident began when his friend Rourke cut in front of the suspect in the snack bar line. Neither he nor Trevino saw the attack that occurred after the last game of the night with about 40 people at the field.
But Trevino, 50, said witnesses told him the two boys teased each other before the suspect pulled a bat from his bag and hit Rourke in the knees, then the head.
Earlier, the suspect had appeared upset about the game but nothing seemed out of the ordinary, said Trevino, a retired rehabilitation therapist. The boy was known for being competitive but did not have a reputation for being violent, he said.
Rourke was white and the suspect is black, but "racism had nothing to do with it," Trevino said. "This is a behavior problem, not a racial problem."
After the attack, Trevino saw the suspect standing against a fence with his parents. "He looked scared. He was in shock," the coach said.
Other onlookers said the 13-year-old seemed dazed after the incident.
"He just didn't realize it, it just happened and before you know it and then like I say, I told him, 'What did you do, why did you do that?"' parent Sam Cordova told Fox 11 News.
"I could see it in his eyes that it was starting to sink in, oh my gosh, what did I do?" Cordova said.
The Palmdale Pony League field has a sign recruiting boys 5 to 14 and encouraging parents to "Protect Our Nation's Youth."
Parent Jorge Morey, 37, said he was stunned that the attack occurred in such a respectable league.
"It's a real family atmosphere. That's what I like about it," he said.
League officials said play would be suspended at the field for several days.
"We're really in a state of shock over this whole thing," league President Ken Curtis said. Rourke had been a junior umpire and his father is a past president of the league.
A memorial was held at Highland High School, where Rourke attended classes. Students shared memories, and a bundle of pink and black balloons was released.
"He's just a funny kid. He liked to make people smile," Gosporra said. "You could be in the worst mood and he would make you laugh."