God is a rock in that He is strong, He is immovable, He is unshakeable, He can protect those who hide behind Him, He is the material from which our defenses are built, He is the high place from which we can defend ourselves from the enemy, He is eternal, He stands when everything man has built crumbles and falls.
Rock is the basic building material, it is the entire landscape in many places (i.e. Ein Gedi where David hid), The top of a large rock formation could be used as a fortress from which to hide or defend oneself or a group (i.e. Masada), Rock is a place to hide someone or something (i.e. Qumran and the Dead Sea Scrolls). It is basically something that is seen constantly in everyday life in Israel.