Mark 15:8

‘And the crowd went up and began to ask him to do as he was wont to do to them.'

The crowd began to ask him to fulfil his custom and make the customary release. But from where did this crowd come? Not from among the pilgrims who had kept the Passover, and having eaten their meal would be resting and preparing for the day ahead, not knowing of the drama that was being carried out. Rather it would come from those in Jerusalem who had a particular purpose in being there because of the custom and because of the men who were being held. They had probably come specifically seeking the freedom of one of the insurrectionists. The placement of Mark 15:7 before this explanation confirms their connection with them. No doubt there would also be a goodly crowd of High Priestly supporters, brought along by the Chief Priests. A crowd was always useful for convincing judges of which way to make their decisions, for they suggested the way that popular opinion was focused. And even in those days popular opinion could not be ignored.

If Barabbas was called Jesus Barabbas, and Pilate overheard the name Jesus being called out, he may well have mistaken it for a popular demand for the release of Jesus and seen this as a way out of his dilemma. This might explain why the choice was finally between Jesus and Barabbas and why Barabbas was favoured.