Pilate, a Roman governor, found himself in a bind. The Jewish leaders had handed Jesus over, but Pilate saw Jesus' innocence. What custom could he use to try and save Jesus' life?
The Roman governors in Judea had a custom tied to the Jewish Passover feast. They would release one prisoner chosen by the crowd. This wasn't necessarily a Roman tradition, but one adopted to appease the Jewish population and maintain political stability.
Pilate, seeing Jesus was innocent and likely fearing the chief priests' envy more than anything, saw this custom as a way out. He probably hoped the crowd would choose Jesus, the innocent Galilean, over a criminal. This custom was a way for Roman authorities to show a form of clemency, but it also gave immense power to the crowd's desires, a power Pilate was about to regret.