And the intensity spilled into the clubhouses afterward as both sides expressed frustrations.
Left-hander Scott Kazmir of the A's got things rolling when he plunked Royals center fielder Lorenzo Cain in the first inning, prompting a warning to both sides, which led to the ejections of Royals pitching coach Dave Eiland and manager Ned Yost.
On Saturday, Royals starter Yordano Ventura had plunked A's third baseman Brett Lawrie, who had slid hard into Royals shortstop Alcides Escobar on Friday, injuring Escobar.
But Kazmir said he wasn't retaliating by hitting Cain.
"It wasn't intentional," Kazmir said. "I think anyone who has a feel for the game, they would know that it's not intentional. I don't think anyone tries to throw at someone's toe or whatever. I tried to throw a fastball in, tried to throw it hard and got a little more on it because he's quick inside, and I just pulled it. That's all it was."
The Royals weren't so sure.
"I'm not sure," Cain said. "He did ask me if I was OK. ... But no one likes getting hit. It never feels good. I definitely didn't like it. But I got a team that will back me up."
Royals starter Danny Duffy wasn't pleased that Cain got hit for the third time this season, or that his teammates have now been hit 14 times.
"I'm tired of him getting hit," Duffy said. "I'm tired of my brothers with bruises.
"We've worn a lot of pitches this year. When you take as many in the ribs or in the legs as we have, it raises eyebrows."
The hostilities escalated in the eighth inning when Royals reliever Kelvin Herrera threw a 100-mph fastball behind Lawrie.
"That ball to Lawrie, that was something I felt like was completely unprofessional," Kazmir said. "You don't play the game that way."
Herrera, who said he "just had a bad grip on it," was ejected. Before he left, though, he glared at the A's dugout and then pointed his index finger to his temple. He said later the gesture meant, "Think about it."
Said Lawrie: "That's what got me hot. That's what got me mad. You don't throw behind someone and then walk away, when you throw 100 mph, and say, 'The next time I face you, I'm gonna hit you in the head.' That's [garbage]. That's some [garbage], and he needs to pay for that. That ain't OK. This is a game. This isn't going up there and trying to hurt people. This guy doesn't throw 85 mph. He throws 100."