The reason for this is that the scribes and the Pharisees wanted a blind devotion to the law and Jesus did not act in that way. He did something on the Sabbath that they thought was prohibited and so they were angry.
In Jewish tradition, the Sabbath was to be kept holy and essentially no work was to be done on that day. There is a story in Mark 2 that shows how prohibited work was. There, Jesus and his disciples were criticized for breaking off heads of grain and eating them on the Sabbath. They were not really working, just picking a bit of food to eat and that was seen as excessive. Therefore, the scribes and Pharisees were really angry with Jesus when he healed the man with the shriveled hand on the Sabbath. They felt that he was breaking the law. The point of this is to illustrate that Jesus was (as is said in Mark) the “lord of the Sabbath” and that he is above petty laws like those that the Pharisees thought were so important.