“And it came to pass, that he went through the corn fields on the sabbath day; and his disciples began, as they went, to pluck the ears of corn.”
One Sabbath Jesus was going through the grainfields, and as his disciples walked along, they began to pick some heads of grain — the Sabbath grainfield controversy is the chapter's fourth and final conflict, shifting the topic from Jesus' associations and practices to his disciples' behavior. Picking heads of grain while walking through a field was explicitly permitted by Torah (Deuteronomy 23:25) — it was not stealing, and no one objects on that ground. The Pharisees' objection is about the Sabbath: picking grain involves a form of work (harvesting) that is prohibited. The disciples' action was not a deliberate provocation but an ordinary act of hunger — and it triggers the chapter's most consequential controversy.
Church Fathers on Mark 2:23
We read also in the following part, that they who came and went away were many, and that they had not time enough to take their food, wherefore, according to man’s nature, they were hungry.
For it was a precept in Israel, delivered by a written law, that no one should detain a thief found in his fields, unless he tried to take something away with him. For the man who had touched nothing else but what he had eaten they were commanded to allow to go away free and unpunished. Wherefore the Jews accused our Lord’s disciples, who were plucking the ears of corn, of breaking the sabbath, rather than of theft.
For David, when flying from the face of Saul [1 Sam 21] went to the Chief Priest, and ate the shew-bread, and took away the sword of Goliath, which things had been offered to the Lord. But a question has been raised how the Evangelist called Abiathar at this time High Priest, when the Book of Kings calls him Abimelech.