It looks like there was a tradition that a prisoner would be released at that time. That would have been a reason for many Jews to be there I would say.
Of historical importance in reconstructing the circumstances of Jesus' death is the fact that Passover in Jerusalem could be a volatile time. Thousands of Jewish pilgrims streamed to Jerusalem from all over the Mediterranean world to celebrate the festival of freedom from foreign domination, but upon arriving they would see many signs of Roman supremacy. The first-century writer Flavius Josephus tells of the regular Roman practice of stationing troops to maintain public order in the Temple precincts (
Jewish Wars, 2.12.1). The inflamed mood of the Jewish populace at Passover probably explains why Pilate was in Jerusalem, instead of at his headquarters in Caesarea Maritima, when Jesus entered the city. If, as the synoptic Gospels relate, Jesus caused a disturbance in the Temple after his arrival, this would certainly alarm both Jewish and Roman authorities: a Galilean troublemaker might be planning to start a Passover riot. Pilate would want to keep the peace. So would Caiaphas, who could reasonably fear that violence could lead to the destruction of the Temple, as indeed eventually occurred (see John 11:48-50).
It is because I accept the account in the gospels
Yet they do not all agree. The "blood curse" in 27:25 is thus not a Stage 1 historic event, but is part of Matthew's Stage 3 polemic against Jewish rivals.
Pilate released Barabbas to them.
An interesting character. Questions mount when one realizes that the name Bar-abbas means "son of the father," an excellent soubriquet for Jesus of Nazareth. In fact, in certain texts of Matthew's Gospel, Barabbas' surname is Jesus and so Pilate asks whether he should release Jesus son of the Father or Jesus the messiah (Mt 27:17)!
One solution;
A man with the name Barabbas was arrested after a riot that had caused some deaths in Jerusalem. Eventually he was released by Pilate when a feast brought the governor to Jerusalem to supervise public order. Presumably this took place at the same time that Jesus was crucified, or not far from it, or on another Passover. In any case, this release struck Christians as ironic: The same legal issue was involved, sedition against the authority of the emperor. Although they knew Jesus was innocent, he was found guilty by Pilate, while Barabbas was let go. The storytelling tendency to contrast the released Barabbas and the crucified Jesus by bringing them together at the same moment before Pilate's 'justice' would have been enhanced if both had the same personal name, Jesus (819-20).
Given the difficulties and uncertainties associated with the Barabbas incident, it questions the degree of historicity.