The PGA Tour announced the decision (via Bob Harig of ESPN), with Reed saying he respected the decision to remove Karain.
Multiple reports indicate that Karain threw a punch:
According to Harig, a source said Karain punched the fan after Reed and Webb Simpson lost their fourball match to Hideki Matsuyama and C.T. Pan.
Karain told ESPN's Michael Collins (via Harig) that the fan said "You f--king suck" to Reed before he reacted:
"As a caddy one of your jobs is to protect your player. And unlike several other sports, in golf fans can get pretty close to Athletes.
"We have been known for having fun with some good banter, but after hearing several fans in Australia for 3 days some had taken it too far, I had enough. And this gentleman was one of them.
"I got off the cart and shoved him, said a couple things, probably a few expletives. Security came and I got back in [the] cart and left. I don't think there's one caddy I know that could blame me."
Karain also told Collins that fans have been using profane language against Reed all week with no consequences.
Reed has been under fire of late after NBC cameras caught him scooping some sand before his ball on practice swings during last week's Hero World Challenge, thereby improving the lie. The PGA Tour assessed a two-stroke penalty.
Reed denied cheating and offered the following explanation:
Some people did not buy that, including Australian golfer Cameron Smith.
"If you make a mistake maybe once, you could maybe understand, but to give a bit of a bulls--t response like the camera angle...that's pretty up there [inexcusable]," Smith told Evin Priest of the Australian Associated Press (via Yahoo).
The Melbourne fans have given it to Reed this week, with Adam Schupak of Golfweek providing some examples on Day 1:
"From the you can't make this up department: Reed hit into bunkers on the first three holes. There were catcalls of 'get in the bunker' and 'stay out of the bunker.' Seemingly every hole, a spectator or two made some crack but it was rarely over the top.
"When Reed drove into the hay right of the seventh fairway, a spectator declared it a bad lie and joked, 'Go ahead and fix it. Take a foot wedge.'
"Reed was heckled with 'Mr. Sandman,' and there were a few cries of 'cheat,' most vocally from a boozy group of fans holding Heinekens in a chalet bordering the 10th hole."
Simpson called the remarks "undeserved."
Reed, the 2018 Masters Tournament champion, will join the rest of his American teammates against the International Team for the final day of the competition, which features 12 singles matches.