The disciples forgot to bring bread along for the trip. This was what was on their minds. So when Jesus mentioned the "leaven" of the Pharisees and Sadducees the disciples assumed Jesus was concerned about the lack of bread too.
Why do we do this? Why do we think God gets wound up about the things we stress about? Provisions are God's department. He can and will provide for us. I don't think He even really cares what we eat or drink as long as what we consume doesn't consume us or take our hearts from Him.
Jesus reminds them of the provision miracles they recently experienced. He's telling the disciples God will provide for them too.
We need to be concerned about what's going on in our hearts, what and who we listen to and follow.