Kelly threw the beanball in the seventh inning of the Red Sox's 10-7 win Wednesday in Boston.
The bad blood boiled earlier in the game, when Austin slid into second base and swept his leg up into Brock Holt in the third inning. Benches also cleared after Holt got in Austin's face, expressing his displeasure over the collision. But by the time reinforcements arrived, the fight had already fizzled.
Kelly wasn't the first pitcher Austin faced after his questionable slide. He battled Heath Hembree in the fifth inning and struck out.
He fooled Austin on a swinging strike on his first offering, before tossing a pitch way inside. His third offering was below the strike zone for another ball. He finally made contact with the Yankees' first baseman on the fourth pitch, a 97.7-mph fastball.
Austin slammed his bat on the ground and took off toward Kelly. The Red Sox pitcher ended up on top of Austin and got in a few hits before both teams got involved in the scrum.
Yankees slugger Aaron Judge stood in the middle of the action, acting as a peacemaker.
"I was ready to defend myself," Kelly told reporters. "Someone comes in my property in my backyard? I have two dogs. Ready to come on my property and I'm being attacked, then I'm ready to defend myself."
Home plate umpire Hunter Wendelstedt ejected Kelly, Austin, Yankees pitcher Tommy Kahnle and Yankees third base coach Phil Nevin for their roles in the scuffle.
"I felt like my slide into second base was a clean slide," Austin told reporters. "I play the game hard. I thought there was absolutely nothing wrong with that slide. I had no thought that they were going to throw at me."
Austin went 1-for-2 with an RBI, a walk and a strikeout in the win. Kelly threw just seven pitches in 0.1 innings of work.
"I thought it was a hard slide into second, nothing remotely dirty about it," Yankees manager Aaron Boone said. "To take matters into your own hands and you go hit one of our guys for that, I thought was an overreaction. I didn't think it was right."
The Yankees and Red Sox resume their storied rivalry at 7:10 p.m. Thursday in Boston in the final showdown of a three-game series. The series is tied 1-1.
Boston and New York have another series coming up on May 8 at Yankee Stadium. The Yankees and Red Sox scuffle wasn't the only dustup of the day in Major League Baseball. Five players were ejected earlier in the day for their role in a bench-clearing brawl during the Colorado Rockies' 6-4 win against the San Diego Padres at Coors Field in Denver.
Here's Tyler Austin's slide from earlier. Spikes come up a bit, but he hardly made contact with Holt. He slides through the base. His job is to break up the play. He did his job and he didn't hurt anyone. No malintent there.#Yankees #RedSoxpic.twitter.com/E5c6FwqpBe— Gary Phillips (@GaryHPhillips) April 12, 2018
Players involved in the altercations are likely to hear from the league regarding disciplinary action.