Jesus disciples whilst walking through the grainfields were plucking and eating the grain. It in not wonr g according to Jewish law to pluck the grain as this is allowed in Deut 23:25, not does the Pharisees accusation seem to be aimed atv their walking on the Sabbath. The crime, from the Pharisees viewpoint, seems to be the plucking of grain on the Sabbath. Sabbath observance in 2nd temple Judaism, as with modern Judaism, is kept with various levels of strictness. The Essenes for example forbid the removal of an animal which has fallen into a pit. (Damascus Document 11:13-14, Matt 12:11, Luke 13:15, The Pharisees were not as strict but still only allowed life-threatening activities to be above th law.
In response to the accusation of lawbreaking Jesus recalls the story of David. Why does Jesus do this? David is a king on the run who breaks the law due to the extreme situation. Likewise Jesus is a man of authority who in one sense is on the run
The focus of the scriptural allusion is not therefore so much on what David did, as on the fact that it was David who did it, and that Scripture records his act, illegal as it was, with apparent approval. The logic of Jesus’ argument therefore implies a covert claim to a personal authority at least as great as that of David.
France, R. T.: The Gospel of Mark : A Commentary on the Greek Text. Grand Rapids, Mich.; Carlisle : W.B. Eerdmans; Paternoster Press, 2002, S. 145
This pericope ends with the saying
The Sabbath was made for man, anot man for the Sabbath. 28 So the Son of Man is lord even of the Sabbath.” Mark 2:27-28
The phrase ‘son of man’ is a frequent refrain with Mark’s gospel
‘Son of Man’ is found in synoptica and John. Outside of the gospels it only appears 7 times (Acts 3x, 2 Thessalonians, Hebrews , Revelation.
The phrase ‘son if man’ seems to have been a titular self designation which was used by the church. Presumably the later church preferred a more exalted title to use of Jesus (Christ, Lord).
In this saying Mark 2:27-28 Jesus seems to be comparing his ministry as the ‘son of man/human being’ to that of Adam. Adam had authority in the garden and the Sabbath existed for his pleasure. Likewise Jesus, as bringer of the new eschaton/new creation is Lord of it. A friend at college is writing a thesis which compares Adam Christology within the Pauline texts with ‘son of man’ in the gospels. He makes a convincing case for it.