What event caused God to lose control of his covenant people before the exodus?
The event was the sin of the brethren—Joseph’s brothers selling him into slavery—which set in motion Israel’s move down into Egypt. That sin precipitated the situation in which God lost control of His covenant people, leaving them under Egyptian idolatry and in need of redemption.
Les explains that when the brothers sold Joseph into slavery and Joseph ended up down in Egypt, it was the sin of the brethren that got the old ball rolling and caused God to lose control of His covenant people. They were then down in Egypt under the influence of all the idolatry of the Egyptians. Because of this, God would have to redeem them, pay the price to bring them back unto Himself, fitting the book of Exodus as a book of redemption.