Villanueva, a former Army Ranger who served terms in Afghanistan, was the lone Steeler to appear despite an expectation from Saturday's players-only meeting that the entire team would remain in the locker room, the source said.
The starting left tackle was a key figure in the team's protest planning because of his background. Players wanted to accommodate Villanueva, who expressed during their 30-minute session that he didn't want to be singled out, the source said. Moving the protest off the field entirely was a way to keep solidarity without isolating an individual.
"That's where the confusion came in," the source of the anthem sequence said.
Players discussed several potential methods of protest at that meeting, including holding hands on the sideline or laying hands on teammates who would stand or kneel. The team eventually moved their locker-room plan into the tunnel for time purposes.
Asked what played into Villanueva's leaving the tunnel, defensive end Cam Heyward said: "I don't want to go into that, but we support our guy Al. He feels he had to do it. This guy served our country, and we thank him for it."
James Harrison told PennLive.com that he did not think anyone was exempt from the pregame plan.
"We thought we were all in attention with the same agreement, obviously," Harrison told PennLive.com. "But I guess we weren't."
In the past, Villanueva has said he aligned with player frustration over racial injustice, but the sacrifice of the military is too great to minimize the anthem.
Teammates widely respect Villanueva for serving the country, and they made that clear Sunday.
"Al is a unique circumstance, what he's been through, some of the things he's talked about before," guard David DeCastro said of Villanueva's decision Sunday.
"I've got a lot of respect for Al. I wish there was a different way to do this thing. We've got some people who look at the national anthem as patriotism, soldiers, all the stuff that it means, and obviously, people are upset, and I understand that. I just wish both sides understand that they want the right thing, but doing it through the national anthem, I wish there was a different way."