A little personal history was at play, as were rules that aren't in any rule book.
Now the Dodgers will be without their $147 million pitcher for several weeks and Quentin is expected to be suspended by Major League Baseball, partly because of baseball culture and its fuzzy, unspoken guidelines on just when and how it's OK to bean someone.
After Quentin got hit, the San Diego Padres' slugger took a few steps onto the grass. When Greinke, Los Angeles' prize off-season signing, appeared to say something, Quentin tossed his bat aside and rushed the mound.
The six-foot-two, 195-pound Greinke dropped his glove and the two players lowered their shoulders. The 6-2, 240-pound Quentin — who as a high school senior was named his league's defensive player of the year as an outside linebacker — slammed into the pitcher.
Quentin and Greinke ended up at the bottom of a huge scrum as players from both sides ran onto the field and jumped in.
Greinke took the brunt of the blow, breaking his left collarbone and inciting a fight that didn't even end when the game was over.
"It's a man's game on the field," Quentin said. "Thoughts aren't present when things like this happen."
Quentin said later that getting plunked by pitches by Greinke during the 2008 and 2009 seasons was justification enough to charge the mound when it happened again.
If Greinke hadn't said anything, "There's a chance I don't" rush the mound, Quentin said. "Like I said, there is a history there, which is the reason I reacted like I did. Who knows what happens if he doesn't say anything or if he motions that it wasn't intentional?"
While pitching for Kansas City against the Chicago White Sox on July 18, 2008, Greinke hit Quentin with a pitch near the left wrist, loading the bases. Then on April 8, 2009, Greinke hit Quentin between the shoulders in the fourth inning after throwing one high and tight during Quentin's previous at-bat. Quentin took about a step toward the mound then, before plate umpire Bill Hohn jumped in front of him.
At its core, Thursday's brawl was about baseball's quirky decorum. The game naturally has a tension between pitchers and batters over balls thrown over the inside of the plate, and sometimes that flares into disagreement over who "owns" the inside half. Even if Greinke simply missed his location on the pitch that hit Quentin, the slugger apparently felt there was intent to hit him.
The Dodgers were vehement that Greinke wouldn't put a runner on base leading off an inning in a one-run game. Greinke's body language momentarily appeared like he regretted that he hit Quentin. But the fact he threw his glove down and said something perhaps indicated that he felt he had the right to the inside of the plate.
Quentin's rushing the mound was taking baseball protocol to the extreme, whereas in many other instances, the batter might gesture and yap while being escorted toward first base by the umpire and the catcher.
Quentin was hit by a pitch above the right wrist by Dodgers reliever Ronald Belisario in Tuesday's series opener and had to leave the game. He sat out Wednesday night's game.
The Dodgers weren't buying Quentin's explanation. So much so that Matt Kemp, among four players ejected after the brawl, confronted Quentin as they left Petco Park following LA's 3-2 win. Big Padres lefty Clayton Richard stepped between the two, and police and security moved in to break it up.
Several Dodgers said Quentin should have known by the situation that Greinke's pitch didn't come with a purpose.
"I got emotional because first off, we shouldn't even have been in that situation," Kemp said in the clubhouse before his confrontation with Quentin. "People with good baseball IQs know that when you have a one-run lead in the sixth inning and it's a 3-2 count, Greinke's not going to hit you on purpose.
"I think Carlos Quentin went to Stanford? Something like that. Yeah. I heard there's smart people at Stanford. That wasn't too smart," Kemp said.
Several Dodgers mentioned how Quentin crowds the plate. Manager Don Mattingly called Quentin "a guy that basically dives into the plate."
Mattingly was livid and Kemp wasn't far behind.
"People were saying in the pile, they've got history," Kemp said. "Come on guys, history? I mean, Greinke's trying to win games. He's not trying to hit anybody on purpose. If you look at the video, if you look at where Quentin actually stands on the plate, he gives the pitcher no space to even come inside. Good pitchers have to come inside. Sometimes they miss their spots. Greinke missed his spot right there. That's when he hit Carlos Quentin. No big deal. Take it like a man and walk down to first base. But he had to charge the mound. Now one of our best pitchers is hurt for no reason."
Mattingly added that Quentin showed "zero understanding of the game."
"He should not play a game until Greinke can pitch. If he plays before Greinke pitches, something's wrong," the Dodgers manager said. "He caused the whole thing. Nothing happens if he goes to first base."
After the teams started going back to the dugouts and bullpens following the brawl, Jerry Hairston Jr. came running across the field yelling and pointing at someone in the San Diego dugout and had to be restrained. Hairston claimed a Padres player — whom he wouldn't name — was making fun of the fact Greinke had been injured.
Hairston, Quentin, Kemp and Greinke were ejected.
Greinke, the 2009 AL Cy Young Award winner, had his left arm in a sling and a dazed look on his face as he told his side of the story.
"I never hit him on purpose," said Greinke, who still appeared shaken after the game. "I never thought about hitting him on purpose. He always seems to think that I'm hitting him on purpose, but that's not the case. That's all I can really say about it."
Greinke was asked if he said something to Quentin before the slugger rushed the mound.
"I'm not really talking about this. I said what I said to you guys."
Asked if there was bad blood between the teams, Greinke said: "Now there probably is. I don't know if there was beforehand."
The teams play another three-game series at Dodger Stadium beginning Monday night.
Greinke said the injury was "awful. It's silly that something could happen like that. I'm disappointed."
Quentin has been hit by pitches 116 times in his career, including an AL-high 23 times in 2011 with the White Sox. Greinke has hit 46 batters since his big league debut in 2004.
"I've been hit by many pitches," said Quentin, plunked more often than any other major league hitter since the start of 2008. "Some have been intentional, some have not been. For the amount I have been hit and my hitting style, I'm going to repeat: I have never reacted that way."