Believing that Jesus was out of his mind, his family wanted to come and grab him. There are a number of reasons as to why they might have felt compelled to do so:
They might have been afraid that if Jesus carries on with his ministry, the Roman authorities or the Jewish leaders would harm him. If they had heard about growing discontentment among the Pharisees, in an effort to keep Jesus from harm, they came to grab him and take him back home to Nazareth.
The family did not believe in Jesus yet. They might also have been worried that they would be ostracized from the Jewish community if they did not do something to stop Jesus. Clearly, they did not truly believe that he was the Messiah and they came to grab him before it was too late, because they tried to save their family from shame when Jesus’ claims would later be shown to be false (as they believed at the time), and when he would be rejected by the religious leaders.
21 And when his family heard it, they went out to seize him, for they were saying, “He is out of his mind.”