Can anyone tell me why the fight between kostya tszyu and leonardo mas was a no contest?

"1997 began for Tszyu when his defense against Leonardo Moro Mas was declared a no contest because Mas' camp protested that the blow that finished their fighter in the first round was actually thrown after referee Joe Cortez had called for a break. Undecided whether it was or was not after Cortez called for a break, the IBF and the Nevada Athletic Commission decided to declare it a no contest instead. For his next bout, however, Tszyu wasn't as lucky, and he lost for the first time, losing by a knockout in ten rounds to Vince Phillips, who also took with that, Tzsyu's world championship."