Police said it was unclear what started a verbal confrontation between the customers and a 20-year-old employee Thursday on Detroit's east side, the Detroit Free Press reported.
When the employee punched the man, the victim "fell to the ground," Sgt. Eren Stephens, a spokeswoman for the Detroit Police Department, said.
"We're now waiting on the medical examiner's report to determine the cause of death," she said. "The 20-year-old has been taken into custody."
Police told WJBK-TV, Detroit, the punch might have caused the victim to choke on his dentures.
A spokeswoman for Burger King told WJBK-TV the franchise owner was cooperating with investigators.