Jesus was a problem to the religious authorities.  He did not fit into their mould or play their religious games.  The Pharisees believed that they had a perfect understanding of (what we know as) the Old Testament and therefore had the right to condemn 'law-breakers'.  They were proud and self-important, unable to keep the law themselves (Matthew 23:1-4), but making up new rules and keeping people away from the grace of God (Luke 11:52).  Jesus' humble authority threatened the Pharisees, who were always looking for a reason to accuse Jesus and kill Him.
 
On the Sabbath, the disciples were going through a cornfield and plucked some ears of wheat.  That was specifically allowed in the Law of Moses (Deuteronomy 23:25).  For the disciples it was eating a little snack because they were hungry.  However, the Pharisees considered that they were 'working' on the Sabbath - reaping and milling as they took the corn and rubbed the husks off the grain.  So they accused Jesus of allowing His disciples to 'break God's law'.
 
Jesus responded by challenging their knowledge of Scripture.  In 1 Samuel 21:1-6 King David and his fighting men were hungry and a priest gave them consecrated bread to eat.  Although the bread was holy to the Lord (and only to be eaten by the priests), it was kind and right to give it to the starving troops.  By that example, Jesus pointed out the weakness of the accusation against Him.  They knew the Law, and made up extra clauses to burden the people.  But they did not understand the purpose of the Law.
 
Legalism can also be a problem in the church of Jesus Christ.  It afflicts proud and arrogant believers who like to make up extra rules to prove their own piety or control other people.  But the truth of God's Word is not intended to shackle the believer; it is designed to set them free to serve the Lord (John 8:32).  Instead of being quick to accuse, the truth teaches us to forgive.  Instead of condemning others and exalting ourselves, the way of Christ is to humble ourselves and exalt others (Romans 12:3).  The problem with self-righteous accusation is that it soon becomes a habit.  Deal with it - before you spoil the grace of God in your own life and everybody else's!