While the Giants warmed up in the outfield, Pham confronted Pederson and smacked him in the cheek before the pair was separated.
"It was a surprise," Pederson told reporters. "There was no real argument. He kind of came up and said, 'Like, I don't know if you remember from last year.' And I was like, 'Fantasy football.' He was like, 'Yeah.'"
Major League Baseball is investigating, and Pham agreed to be scratched from Cincinnati's lineup shortly before first pitch pending results of the inquiry. He also was not in the lineup for Saturday's game.
Pederson said after the Reds' 5-1 victory that he was accused of cheating for placing a player on injured reserve and replacing him with a free agent in a fantasy football league. Pederson said the player he put on IR had been ruled out for that week, which made it a legal move. He said Pham had executed essentially the same maneuver with his own team.
"I sent a screenshot of the rules, how it says that if a player's ruled out, you're allowed to put him on the IR and that's all I was doing," Pederson said. "He literally did the same thing. That was basically all of it."
He continued by saying that he thought the fantasy player at the center of the disagreement, San Francisco 49ers running back Jeff Wilson, might have been a player Pham had in two different leagues, leading to a possible mix-up.
"Maybe that was a confusion," he added. "In the ESPN league we were in, he was listed as out. It feels very similar to what I did. That was basically all of it. There's not much more to it."
Pederson said he had no advance notice that Pham might confront him during the series in Cincinnati.
The Giants outfielder said he didn't retaliate after being slapped, and he has no plans to reengage Pham while the team is in Cincinnati.
"Violence isn't the answer. It's over as far as I'm concerned," Pederson said. "I won't talk to him. I don't think he wants to talk to me, I don't know. It was a weird interaction."
Pham threatened violence to settle an on-field score with San Diego's Luke Voit in April, challenging the slugger to a fight after Voit injured Reds catcher Tyler Stephenson with a hard, ugly slide into home.
"If Luke wants to settle it, I get down really well," Pham said. "Anything. Muay Thai, whatever. Like I said, I've got an owner here who will let me use his facility."
The Pham-Pederson scuffle was witnessed by reporters and occurred before fans entered the ballpark.
Reds manager David Bell refused to comment on the incident, and Pham refused to speak on the record with reporters.
"Major League Baseball is investigating it, and until that's complete, I'm probably not going to say much on it," Giants manager Gabe Kapler said.
The Associated Press contributed to this report.