It looked like the disciples had been so busy
ministering to the crowds, that they had
forgotten to bring enough bread.

While Jesus was in the ship, he was thinking
about the Pharisees and their wanting a sign
from heaven. He decided that He must warn
the disciples of them. So He said, "Take heed,
beware of the leaven of the Pharisees, and
the leaven of Herod". Mark 8:15

Some of the disciples didn't get the meaning,
and though He was cautioning them not to
buy bread from the Pharisees. They had
only one loaf with them. Surely after Jesus,
having just fed thousands on little, they weren't
afraid of starvation!