After Nowitzki grabbed a rebound on the defensive end, Hollins threw his left forearm at Nowitzki's head, landing a glancing shot. Officials reviewed the play and assessed a Flagrant 2 foul, which brought the ejection.
It was the second confrontation of the night for Hollins. He appeared to accidentally put an elbow into DeShawn Stevenson's head during a scrum for a rebound in the first quarter. Stevenson reacted strongly and was assessed a technical foul before confronting Hollins, who was not charged with a foul on the play.
"Hollins is a little out of control, but hey, it happens out there," Nowitzki said with a shrug.
Hollins was not available for comment after the game, but coach Kurt Rambis said he didn't think the act was malicious.
"I don't like some of his decision-making, but I like his aggressiveness," Rambis said. "When guys play hard, there's a lot of physical contact, tempers are going to flare."
Hollins had 13 points and nine rebounds in the game.
Information from The Associated Press contributed to this report.