On the first day of the last week of his mortal life, Jesus entered the capital city, Jerusalem, seated on an ass. It was a political act. So the crowds are lauding Jesus as the king whom God has appointed to re-establish Israelite independence and military prowess. They honor Jesus by paving his path with tree branches and their own clothes. Crowds always honor victors.
Thousands turn out each year to honor the super bowl or world series champions.
When the blood gets up, it is hard to contain a crowd's exuberance. The crowd shouted for the release of Barabbas instead. Barabbas had actually done the sort of thing Jesus was accused of. The Jewish clergy, who should have known, incited the crowd to acclaim Barabbas. He had the glory of having started a rebellion.