Jamieson, Fausset and Brown on Genesis 35:5

5. the terror of God was upon the cities--There was every reason to apprehend that a storm of indignation would burst from all quarters upon Jacob's family, and that the Canaanite tribes would have formed one united plan of revenge. But a supernatural panic seized them; and thus, for the sake of the "heir of the promise," the protecting shield of Providence was specially held over his family.