NOTES for Mar 8:14-21
Jesus' disciples were mostly simple people, and therefore they understood His words so literally. They hear about leaven and immediately decide that He is speaking about a shortage of food. It appears to them that the Lord is reproaching them, that He is displeased because they did not calculate the food for themselves and the people. But Christ's words were figurative, offering an image of the Pharisaism that could begin to grow in them. This is what He was warning them against.
The Savior's words are sharp enough. He really is reproaching them, but not because there is no bread. He reproaches them because they do not remember the miracle of multiplication that happened so recently. It turns out that they are so concentrated on themselves, on the things for which they blame themselves, that they do not pay attention to what is happening around them and forget the miracle in an instant.