Adam Clarke on Genesis 31:42

The fear of Isaac - It is strange that Jacob should say, the God of Abraham and the Fear of Isaac, when both words are meant of the same Being. The reason perhaps was this; Abraham was long since dead, and God was his unalienable portion for ever. Isaac was yet alive in a state of probation, living in the fear of God, not exempt from the danger of falling; therefore God is said to be his fear, not only the object of his religious worship in a general way, but that holy and just God before whom he was still working out his salvation with fear and trembling, fear lest he should fall, and trembling lest he should offend.