Mark 2:24

‘And the Pharisees said to him, “Look, why do they on the Sabbath day what is not lawful?”

They were probably quite genuinely upset. There is no one more vulnerable than the sincere person who has established a set of regulations as being right and then sees them being flouted. They just could not understand how Jesus, Who seemed willing to live within their requirements (which they saw as God's requirements), could allow such a thing to be done. They considered that the disciples were behaving unlawfully with great abandon. They were defiling God's day of rest.

But the statement may be a little more sinister than that. The punishment for Sabbath breaking was stoning, and certainly later it was laid down that a warning must first be given before the stoning could take place. Men must be given one chance. Thus ‘what is not lawful' may have been an official warning. They may have been saying, ‘we are giving them a last chance. If they do it again they will be punished by the synagogue.'