The reason there is a custom to bake challot in the shape of a key in honor of the Shabbat after Passover is because at this time of year the Jews entered the Holy Land and the manna stopped falling from heaven. Then they began eating from the produce of the land and didn't rely on a miracle. The key-shaped challah symbolizes the key to livelihood which is in G-d’s hand, and our prayers to Him to open the gates of livelihood for us.
