Morton did not field Arenado's bouncer cleanly. But because of the batsman's non-hustling start, Morton's throw beat Arenado at first on a close play.
"We've talked a lot about playing the game hard and playing the game right, and how important that is," Weiss said. "Especially when you've been getting your [butt] kicked for a while, it's even more important. It's imperative. So I just felt like Nolan didn't meet that standard at that point.
"Now, Nolan's a great kid. He's a great player. That's how I saw it."
Weiss said after the game he had not talked with Arenado, who was 0-for-2 at the time of the benching and saw his hit streak end at five games.
Arenado won a Rawlings Gold Glove Award last season as a rookie -- the first National League rookie third baseman and second overall at his position to win the award -- and set a club record with a 28-game hit streak earlier this year. But Arenado also missed 37 games in May and June with a fractured left middle finger, and has just recently been regaining his timing at the plate.
Arenado was not available after the game, but apologized through a club official and said he would address the incident Saturday.
Thomas Harding is a reporter for MLB.com. Read his blog, Hardball in the Rockies, and follow him on Twitter @harding_at_mlb. Cody Ulm is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.