Newman said he bumped into Stewart a little bit after Stewart cut him off and, according to Newman, intentionally drove across the nose of his car.
"I don't think there was any reason other than him just being bipolar and having anger issues," Newman said. "Google 'Tony Stewart.' You'll see all kinds of things he's done. Look it up on YouTube."
Stewart is in his final Sprint Cup season before hoping to return to sprint car racing, in which he will pick and choose the races he will enter.
Newman, who was having a frustrating day and not appearing to be able to make up the 22 points he needed on Jamie McMurray to make the Chase for the Sprint Cup, took a shot at Stewart, who was involved in a tragedy in August 2014, when he struck and killed Kevin Ward Jr. in a sprint car race. Stewart was not criminally charged but faces a wrongful death lawsuit.
Stewart claims he never saw Ward until the last moment. Ward had approached Stewart's car on foot, and Stewart claims that Ward tried to grab his car while under the influence of marijuana.
"I guess he thought he was in a sprint car again -- didn't want to control his anger," Newman said. "We'll keep fighting like we always do. It's unfortunate. Not the end we wanted.
"[It's] just disappointing that you have somebody old like that that should be retired the way he drives. It's just ridiculous."
Newman drove for Stewart-Haas Racing from 2009-13.
"He's got issues, and we all know he's got issues," Newman said in reference to Stewart. "He proved it again tonight. I was clearly inside of him getting into Turn 1, he cut across my nose, I was on the brakes, on the apron and I hit him coming off of Turn 2 but only because I got loose [and] I was on the apron.
"[It's] just disappointing that you have somebody old like that that should be retired the way he drives. It's just ridiculous."Ryan Newman on Tony Stewart
"The next thing I know, he is driving across my nose on the back straightaway because he's Tony Stewart. He thinks he owns everything. It's unfortunate. But we shouldn't expect anything less from him."
Stewart said Newman had run across his nose early in the race and then twice again before Stewart admitted he cut across Newman but obviously didn't want to wreck himself.
"How many times does he get to run into you before you say, 'You've had enough?" Stewart said.
As far as the harsh comments, Stewart said Newman was in a pressure-filled situation.
"It's a stressful night for him," Stewart said. "He was trying to make the Chase. It's a stressful moment. He's going to say whatever he's going to say."
The two drivers had been friends.
"I'd say by that, he probably doesn't want to be friends right now," Stewart said. "It's up to him."