Mark 15:11

‘But the chief priests stirred up the crowd that he should rather release Barabbas to them.'

The crowd that had arrived seeking the release of Barabbas now gained the support of the Chief Priests, who had now realised that Pilate was making a concession to their demands and that they were winning. So the Chief Priests and their supporters allied themselves with those in the crowd who wanted the release of Barabbas, who were probably both delighted and surprised to get such powerful support, and pressed them to demand Barabbas. It was pure political manipulation.