The incident took place Saturday at the Mineral County Fair in Superior, Montana.
Authorities arrested Brockway and charged him with assault on a minor, which is a felony.
Jasper said Brockway is an Army veteran who believes he was acting on an order by his commander in chief. The anthem was interrupted by sudden "pop" she heard. Brockway allegedly told encroaching bystanders the assault was justified because the boy had been "disrespecting the national anthem", Hennick said.
"There was a little boy lying on the ground", 23-year-old Taylor Hennick told the Missoulian. "He was bleeding out of his ears, seizing on the ground, just not coherent".
The U.S. flag code says people should face the flag during the national anthem.
Jasper said Brockway expressed remorse the child was hurt. The boy was flown by air ambulance across state lines to a Spokane, Washington children's hospital for treatment after the alleged August 3 assault at the Mineral County fairgrounds. The Missoulian reported that the child sustained a fracture to his skull.
After the assault, the boy was taken to a local hospital to be examined.
The boy's parents told MTN that he was bleeding from his ears for almost six hours, after being rushed to a hospital in Spokane, Washington, on the evening of Saturday, Aug. 3. "I don't remember anything - the rodeo - the helicopter - nothing", the 13-year-old said in an interview with the station. "He targeted [my son], grabbed him and took him down".
The teen's family has declined to comment.
The teenager has since been released from the hospital, according to the parents. Trump once called for National Football League owners to fire players who kneel or engage in other acts of protest during the anthem.
"My best friend's son was brutally attacked by an adult Saturday evening in Superior MT while he was entering the rodeo for no reason at all", she wrote. The reason? The boy apparently did not remove his hat during the national anthem. "Maybe they shouldn't be in the country", he said.
"Whether that passes muster with the court as a viable defense is for a different day", Jasper told the Missoulian. Jasper likened Trump's words surrounding the anthem and those who protest it to a presidential order. "I am certain of the fact that [Brockway] was doing what he believed he was told to do, essentially, by the president".
Jasper said that the "heightened rhetoric" contributes as a reason for such things. Brockway's lawyer later cited President Trump's rhetoric as a direct influence on his client's behavior.
This most recent incident came just six months after Brockway was released from supervision from a 2010 case where he was found guilty of assault with a weapon.
The Montana Department of Corrections considered Brockway low risk. He was granted early release from the probation after seven years for good behavior.