North Carolina senior point guard Joel Berry II punched a door after losing a video game to teammate Theo Pinson and a team manager.
That is what caused his broken right hand, which will sideline the preseason All-American for close to four weeks, as previously reported.
"He did a silly thing," North Carolina coach Roy Williams told ESPN on Wednesday in revealing the cause of the injury. "That's what it was. One of his greatest characteristics is his competitiveness."
Berry, who averaged 14.7 points and 3.6 assists last season, was the Most Outstanding Player in last year's Final Four, leading the Tar Heels to a national championship.
North Carolina, No. 9 in the preseason coaches poll, opens the 2017-18 season Nov. 10 at home against Northern Iowa.