Obviously, these Pharisees were offended by the ruckus Jesus and his disciples were making. They were upset with the tumult of the parade into the city. The reference to peace in heaven made them anxious on earth. The concept of peace in the abstract sounds placid and innocuous. But when that concept of peace takes on flesh in human society, it precipitates strife and conflict.
Peace is wonderful as a divine theory.
But when that theory shows up in the concrete, it is not so wonderful after all, especially to those who find their earthly power and authority and their control over others challenged by that peace. What Jesus was saying was that the peace of heaven embodied in him affected not only people, but the very earth itself. Not only was human society addressed by his presence as the Prince of Peace, but the land and the sea, plants and animals, and yes, even the inanimate rocks themselves resonated with the good news of God's love and mercy toward all he had created. What Jesus represented was far greater than a handful of people shouting along the road into the city. What Jesus represented was a new relationship between heaven and earth, which would bring peace to those who needed and would accept it, but distress and anger to those who rejected it.