‘But certain of the Pharisees said, “Why do you do what is not lawful to do on the sabbath day?”
Some of the Pharisees became aware of what His disciples were doing. It may be that they had been walking with the disciples, professing interest in Jesus' message, while carefully watching for any failures in the behaviour of Jesus and His disciples, or it may be that it had simply been reported to them by people who saw it, bringing them hurriedly to the scene. Either way they pointed out that He and His disciples (as their Master He was responsible) were doing what was not lawful on the Sabbath Day.
‘What is not lawful to do.' We should note that this is probably not just a comment. It is an official warning. Proceedings could not be taken under the Law at the first offence. The culprits had first to be warned so as to ensure that they did know what the Law was. If the warning was then ignored, proceedings could be taken. (compare Acts 4:18 with Luke 5:17). Thus Jesus and His disciples were being warned that if it happened again proceedings would be taken. The opposition was hardening.